PK Methodology: How Our Calculator Works

Open Calculator PK Basics Guide Dosing Scenarios Reconstitution Calculator Titration Calculator

Key Takeaways

Model: a one-compartment model with first-order absorption and first-order elimination (the Bateman function). The chart plots the modelled amount of drug in the body, in milligrams.
Inputs per drug: half-life, time to peak, and bioavailability. The calculator applies the time to peak to a single dose and works out the absorption rate from it; the rate is not copied from a paper.
Sources: the FDA labels for semaglutide and tirzepatide, and Phase 1 studies for retatrutide's half-life and time to peak. Two values are chosen where no published figure exists, and are marked as assumptions: retatrutide's bioavailability, and semaglutide's time to peak (the midpoint of the label's range).
One model: for retatrutide and the injectable forms of semaglutide and tirzepatide (Ozempic and Wegovy injection, Mounjaro, Zepbound), every GLP3Planner calculator that draws a drug-level curve, and every dosing scenario page, uses this model with the same parameters. There is no user setting to switch to a different model. The calculator cannot model the tablets: a tablet dose entered as semaglutide, Ozempic or Wegovy is treated as a weekly injection and the chart will be wrong, and an entry named Rybelsus falls back to simple half-life decay.
Concentration estimate: the Hourly Drug Levels table also shows an estimated plasma concentration in mg/L: the modelled amount in the body divided by the drug's true volume of distribution, which is bioavailability times the apparent volume (F × V/F) from the label or Phase 1 data. It is a population average, not a measurement, and not adjusted for body weight.

How the Calculator Works

GLP3Planner treats each injection as a dose that is absorbed from under the skin at a steady fractional rate and removed by the body at a rate set by the drug's half-life. The absorption rate is chosen so that a single dose peaks at the drug's time to peak. For a schedule, the calculator works out each dose's curve and adds them together, which shows how levels build up with weekly dosing and where they level off. The result is a modelled amount of drug in the body, not a blood test result.

Which Tools Use This Model

This page documents what the site's tools actually compute. Not every tool models drug levels.

Tool	Model	Time step
Half-life calculator, the per-compound and brand calculators (Ozempic and Wegovy injections use the semaglutide values; Mounjaro and Zepbound use tirzepatide's), and shared schedules	First-order absorption (this page)	30 minutes (coarser only for very long schedules)
Missed-dose, washout and split-dose calculators	First-order absorption, same parameters	1 hour
Dosing scenario pages	First-order absorption, same parameters (see below)	1 hour
A medication that is not in the parameter table, such as one you add yourself	Simple half-life decay from the half-life you enter, with no absorption phase	Same as the chart
Units, reconstitution, titration, cost, storage and pen-to-vial tools	No drug-level model: arithmetic and dose ladders (the reconstitution and titration rules are below)	n/a

The Pharmacokinetic Model

One Compartment, First-Order Absorption and Elimination

The model treats the body as one well-mixed compartment fed by a depot under the skin. After a subcutaneous injection:

Absorption: drug moves from the injection site into the body at a rate proportional to what is still left at the site. The constant is the absorption rate, k_a.
Elimination: the body removes drug at a rate proportional to the amount present. The constant is the elimination rate, k_e, set by the half-life.
Bioavailability: only the fraction F of the dose ever reaches the body. The rest is lost at the injection site.

With half-lives of 5 to 7 days, all three drugs are removed slowly, which is why once-weekly dosing works and why levels build up over the first weeks of a schedule.^[1][4][7]

The Equation the Calculator Runs

For one dose, the modelled amount of drug in the body t days after the injection is the Bateman function:

A(t) = F × D × k_a / (k_a − k_e) × (e^−k_et − e^−k_at)

A(t) = modelled amount of drug in the body (mg), t days after the dose

F = bioavailability (fraction of the dose that reaches the body)

D = dose (mg)

k_e = elimination rate constant = ln 2 / half-life (per day)

k_a = absorption rate constant (per day), solved from the time to peak

t = days since the dose

The curve starts at zero, rises while absorption outpaces elimination, peaks, and then falls at the rate the half-life sets. Three consequences are worth knowing:

It is an amount, not a concentration. The chart shows milligrams in the body, so the curve needs no volume of distribution and does not depend on body weight. A concentration is estimated separately, for one table only (below).
F scales the whole curve. Bioavailability changes the height of the curve, but not its shape, its timing or the ratio of peak to trough.
Total exposure does not depend on k_a. The area under the curve is F × D / k_e. Faster or slower absorption moves the peak and changes its height, but not the total.

How the Absorption Rate Is Set

In this model a single dose peaks at T_max = ln(k_a / k_e) / (k_a − k_e). The calculator takes each drug's time to peak (T_max) and half-life and solves this equation for k_a, so the modelled peak of one dose lands exactly at that time to peak. The resulting values are in the parameter table.

The absorption rate is derived this way, not taken from a paper, for a reason. Published population models report slower absorption rate constants: 0.0253 to 0.0296 per hour for semaglutide (Overgaard 2019)^[8] and 0.0373 per hour for tirzepatide (Schneck 2024, with F fixed at 0.8).^[9] Plugged into a one-compartment model like this one, those values would put a single dose's peak at about 3.2 to 3.6 days for semaglutide and 2.5 days for tirzepatide, later than the labels report (1 to 3 days for semaglutide, a median of 24 hours for tirzepatide). A one-compartment model can match the label's time to peak or the published absorption rate, not both. GLP3Planner matches the time to peak.

The Concentration Estimate

The chart never uses a volume. One table does: the Hourly Drug Levels table in the calculator (and the same data in the API and the AI-agent tool) shows an estimated plasma concentration in mg/L next to the amount. Labels and trials report the volume of distribution as an apparent volume, V/F: the volume as measured after an injection under the skin, which is the true volume divided by bioavailability. So the true volume is F × V/F, and:

estimated concentration (mg/L) = amount in the body (mg) ÷ (F × V/F in litres)

In words: the modelled amount in the body divided by bioavailability times the apparent volume. The amount already includes F, so F cancels out: retatrutide's assumed bioavailability has no effect on its concentration estimate. The V/F values are in the parameter table.

The estimate is a population average: one volume per drug, not adjusted for body weight or anything else about you. It is not a measurement and cannot stand in for a blood test. As a check, for semaglutide 2.4 mg once a week the model's average over one steady-state week is 0.28 mg/L, about 67 nmol/L; the Wegovy label gives "approximately 75 nmol/L".^[4] A medication that is not in the parameter table has no published volume here, so the table shows no concentration for it.

Drug Parameters

These are the values the calculators use. Each cell says where the value comes from: published (read in the label or paper cited), assumption (a value GLP3Planner chose where no published figure exists or the source gives only a range), or derived (calculated from the other rows). They apply to the weekly injection only.

Parameter	Retatrutide	Semaglutide	Tirzepatide
Half-life	6 days Published: Coskun 2022, Urva 2022^[1][2]	7 days Published: "approximately 1 week", Wegovy and Ozempic labels^[4][5]	5 days Published: Mounjaro label; Zepbound label gives 5 to 6 days^[6][7]
Time to peak (T_max)	1 day (24 hours) Published: median time to peak at 1.5 to 12 mg in the Phase 1 trials (Coskun 2022, Table 1; Urva 2022, appendix Table S3)^[1][2]	2 days Assumption: midpoint of the labels' "1 to 3 days"^[4][5]	1 day (24 hours) Published: Zepbound label median, range 8 to 72 hours^[6]
Bioavailability (F)	80% Assumption: no published value; borrowed from tirzepatide	89% Published: Wegovy and Ozempic labels^[4][5]	80% Published: Mounjaro and Zepbound labels^[6][7]
Apparent volume of distribution (V/F), used only for the concentration estimate	7.4 L Derived: average of the six dose-group values in Coskun 2022, Table 1 (5.85 to 8.62 L, single doses in healthy volunteers)^[1]	12.5 L Published: Wegovy and Ozempic labels^[4][5]	9.7 L Published: Zepbound label, people with overweight or obesity (Mounjaro: 10.3 L in type 2 diabetes)^[6][7]
Elimination rate k_e = ln 2 / half-life	0.116 per day (0.0048 per hour) Derived	0.099 per day (0.0041 per hour) Derived	0.139 per day (0.0058 per hour) Derived
Absorption rate k_a	3.54 per day (0.147 per hour) Derived from T_max and half-life	1.44 per day (0.060 per hour) Derived from T_max and half-life	3.31 per day (0.138 per hour) Derived from T_max and half-life
Status	Investigational (Phase 3), no FDA-approved label	Approved (Ozempic, Wegovy)	Approved (Mounjaro, Zepbound)

The semaglutide values are for the weekly injection. The Wegovy and Ozempic tablets and Rybelsus are absorbed very differently: the labels give about 0.4% to 2% bioavailability and a peak about 1 hour after a dose.^[4][11] The calculator cannot model the tablets: a tablet dose entered as semaglutide, Ozempic or Wegovy is treated as a weekly injection and the chart will be wrong, and an entry named Rybelsus falls back to simple half-life decay.

Notes on Each Value

Retatrutide half-life: in the first-in-human study (single doses, healthy volunteers), "the mean half-life was approximately 6 days", with dose-group averages of 134 to 165 hours (5.6 to 6.9 days).^[1] The Phase 1b study in type 2 diabetes (multiple doses) also reports about 6 days in its abstract.^[2]
Tirzepatide half-life: the Mounjaro label says "approximately 5 days".^[7] The current Zepbound label (section 12.3, August 2026) says "approximately 5-6 days" in people with overweight or obesity, while its overdose section still says about 5 days.^[6] The calculator uses 5 days.
Time to peak and a single dose: the labels do not say whether their times to peak were measured after one dose or at steady state. The calculator applies them to a single dose, which is a modelling choice.
Semaglutide time to peak: both labels say "Maximum concentration of semaglutide is reached 1 to 3 days post dose" and give no median.^[4][5] The calculator's 2 days is the midpoint of that range, an assumption.
Retatrutide time to peak: after a single dose in healthy volunteers, the median time to peak was 14, 12 and 24 hours at 3, 4.5 and 6 mg.^[1] In people with type 2 diabetes taking weekly doses, the medians at week 12 were 12.17, 24.00 and 23.92 hours at 3, 6 and 12 mg (appendix Table S3 of the Phase 1b study).^[2] The calculator uses 24 hours, in line with the week-12 medians at 6 and 12 mg (24.00 and 23.92 hours; 12.17 hours at 3 mg). Two caveats: in the Phase 1b study blood was sampled mostly 12, 24, 48 and 72 hours after a dose, so the true peak lies roughly between 12 and 36 hours; and lower doses (0.1 to 1 mg) peaked later, at 48 to 72 hours.
Retatrutide bioavailability: no absolute bioavailability figure has been published. On 23 October 2026, PubMed and Europe PMC searches and the full texts of the first-in-human study, the Phase 2 obesity trial and a Phase 1b gastric-emptying paper turned up none; the full text of the Phase 1b study could not be reached. Lilly ran a study with an intravenous part (NCT05959096), which is the design used to measure bioavailability. It completed in July 2024, and no results were posted on ClinicalTrials.gov as of 23 October 2026.^[3] The calculator assumes 80%, borrowed from tirzepatide, a related Lilly peptide. This assumption scales the height of the retatrutide curve only.
Absorption rate: derived as described above. Expressed as an absorption half-life (ln 2 / k_a), it is about 5 hours for tirzepatide and retatrutide and 12 hours for semaglutide.
Volume of distribution (V/F): used only for the concentration estimate, never for the chart or any milligram figure. Labels and trials give it as an apparent volume in total litres, not per kilogram. Retatrutide's first-in-human study reports 5.85 to 8.62 L across six single-dose groups;^[1] the calculator uses their average, 7.4 L. For tirzepatide it uses the Zepbound label's 9.7 L for people with overweight or obesity, not the Mounjaro label's 10.3 L for type 2 diabetes.^[6][7] Semaglutide's 12.5 L is the same in the Wegovy and Ozempic labels.

How to Read Your PK Chart

The calculator's chart plots the total drug amount in the body, in milligrams. The figure below shows what the model does with a single 1 mg dose of each drug.

Modelled amount in body after one 1 mg dose of retatrutide, semaglutide and tirzepatide over 21 days

Output of the calculator's model for one 1 mg dose. Each peak lands at the drug's time to peak. The heights depend on bioavailability (89% semaglutide, 80% tirzepatide, 80% assumed for retatrutide) and on how fast each drug is absorbed relative to how fast it is removed.

The Curve

The modelled total amount of drug in the body over time, with bioavailability already applied. It is not a blood level and cannot be compared with a lab result.

Peaks

One dose peaks at the time to peak in the table: 1 day for tirzepatide and retatrutide, 2 days for semaglutide. With weekly dosing at steady state the peak comes a little sooner after each injection, about 0.85, 0.83 and 1.5 days respectively, because the leftover drug from earlier doses is already falling while the new dose is still rising.

Troughs

The lowest points, at the moment of the next injection: absorption is fastest right after a dose, so the curve turns upward straight away. If troughs are low relative to peaks, the pattern is a wide sawtooth, which a split schedule can flatten.

Stability Reached

The chart takes the average modelled amount on each dose day and compares it with the dose day one dosing cycle earlier, checking dose by dose. The cycle length comes from the detected dosing pattern: one dose longer than the repeating pattern of gaps between doses, so two doses for a weekly schedule and three for a Monday and Thursday schedule. The chart marks "Stability Reached" at the first dose day from which these comparisons stay within 3% for a full cycle. If that never happens, a looser pattern check is used; on a Monday and Thursday schedule the three-dose cycle lines up Mondays with Thursdays, so the looser check is usually the one that marks it. This is a chart feature, not a clinical definition of steady state.

Dose Markers

Markers show when each injection happens, so you can see when a dose change starts and how the curve shifts after it.

Multi-Dose Accumulation and Steady State

With half-lives of 5 to 7 days, each weekly dose arrives before the previous one has cleared, so drug accumulates. The calculator handles this by superposition: at every time point it computes what each earlier dose still contributes, using the equation above, and adds the contributions up. The main chart does this every 30 minutes; the missed-dose, washout and split-dose calculators and the dosing scenario pages do it every hour.

Steady state is reached when the amount removed between doses equals the amount added. What the sources say:

Tirzepatide: "Steady-state plasma tirzepatide concentrations were achieved following 4 weeks of once weekly administration" (Mounjaro and Zepbound labels).^[6][7]
Semaglutide: "Steady-state exposure is achieved following 4 to 5 weeks of once-weekly administration" (Ozempic label, type 2 diabetes).^[5] The Wegovy label gives steady-state concentrations but no number of weeks.^[4]
Retatrutide: no measured figure. The first-in-human study used single doses, and the Phase 1b protocol says only that steady state "is expected to be reached in approximately 4 to 5 weeks".^[2] The calculator pages use about 4 weeks (estimated from the ~6-day half-life): in the model the level before the 5th weekly dose (day 28) is 96% of steady state.

What the calculator's model gives for the same dose once a week: the level just before the 5th dose is 98% (tirzepatide), 96% (retatrutide) and 94% (semaglutide) of its eventual steady-state value. At steady state the peak is 2.25 (tirzepatide), 1.97 (retatrutide) and 1.61 (semaglutide) times the trough. The model has no built-in steady-state constant; these figures come out of the superposition itself.

The Dosing Scenario Pages Use the Same Model

The dosing scenario pages run the same engine as the calculators, with the same parameters: first-order absorption, bioavailability, and each drug's half-life and time to peak. They step through each schedule in 1-hour steps, like the missed-dose, washout and split-dose calculators, so a scenario's figures match what those tools show for the same schedule.

Peak and trough: the highest and lowest modelled amounts in the last week of each dosing phase, from that week's first injection up to the next week's.
Washout: the first whole day after the last injection on which the modelled level is below 3% of the peak that follows it (about 97% gone). The level keeps rising for a day or so after the last injection while that dose is absorbed, so the 3% line is measured against that later peak. The washout calculator reports the same day. Five half-lives of pure decay (0.5⁵ = 3.125% left) is half-life arithmetic, not the model's result: in the model the 3% day comes 2 to 3 days later (day 27 tirzepatide, day 32 retatrutide, day 38 semaglutide), mostly because the last dose is still being absorbed.

Until 23 October 2026 the scenario pages used a separate, simpler calculation: each dose counted in full the moment it was injected, with no absorption phase and no bioavailability, in 1-day steps. That put the peak of each scenario's last dosing phase about 30% to 41% above the calculators' for the same dose.

Reconstitution Math (BAC Water → Syringe Units)

A compounded or research vial has to be reconstituted before use: the powder is dissolved in bacteriostatic (BAC) water so a measured dose can be drawn into a syringe. GLP3Planner's reconstitution calculator and dose-to-units calculator use the same three-step arithmetic. It is unit conversion, not a clinical claim.

Step	Formula	What it gives you
1. Concentration	concentration (mg/mL) = vial (mg) ÷ BAC water (mL)	How strong each mL of the mixed vial is.
2. Injection volume	volume (mL) = dose (mg) ÷ concentration (mg/mL)	How much liquid holds your dose.
3. Syringe units	units (U-100) = volume (mL) × 100	The mark to draw to on a U-100 insulin syringe.

The three steps collapse into a single expression:

units = (dose_mg ÷ (vial_mg ÷ bac_mL)) × 100

Worked Example

Reconstituting a 20 mg retatrutide vial with 3 mL of BAC water to draw a 2 mg dose:

Concentration = 20 mg ÷ 3 mL = 6.67 mg/mL
Volume = 2 mg ÷ 6.67 mg/mL = 0.30 mL
Units = 0.30 mL × 100 = 30 units on a U-100 syringe

Conventions Used

U-100 syringe: 100 units = 1 mL. This is the standard insulin-syringe marking, so multiplying the injection volume (mL) by 100 reads straight off the barrel. The markings are units, not International Units (IU).
BAC water: bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative, which lets a multi-dose vial be used over several weeks. The BAC volume only changes concentration. It does not change the total mg of drug in the vial.

For a step-by-step walkthrough with mixing and storage notes, see the retatrutide reconstitution guide, or run your own numbers in the reconstitution calculator.

Standard Titration Ladders

Titration means starting low and stepping the dose up on a fixed schedule so the body can adjust. Each ladder below steps up every 4 weeks, and the final rung is held as the maintenance dose. These are the reference schedules behind the titration calculator, which lets you pick any start and target rung to build a week-by-week plan.

These ladders are educational references drawn from the cited primary sources, not personalized prescriptions. Retatrutide is investigational and has no FDA-approved titration schedule. Always follow a clinician's plan for your own dosing.

Retatrutide^[10]

Start 2 mg → target 12 mg. Investigational: this follows the escalation of the 12 mg group in the Phase 2 obesity trial (2, 4, 8, then 12 mg), not an approved label.

Step	Dose (mg)	Weeks at step	Notes
1	2	Weeks 1–4	Starting dose
2	4	Weeks 5–8
3	8	Weeks 9–12
4	12	Week 13 onward	Target / maintenance

Tirzepatide^[6][7]

Start 2.5 mg → up to 15 mg. Matches the Mounjaro/Zepbound label escalation.

Step	Dose (mg)	Weeks at step	Notes
1	2.5	Weeks 1–4	Starting dose (not a maintenance dose)
2	5	Weeks 5–8	First maintenance option
3	7.5	Weeks 9–12
4	10	Weeks 13–16
5	12.5	Weeks 17–20
6	15	Week 21 onward	Maximum dose

Semaglutide (Wegovy weight-loss ladder)^[4]

Start 0.25 mg → 2.4 mg, the Wegovy label's recommended maintenance dose for weight reduction. This is the Wegovy schedule, not Ozempic's type 2 diabetes ladder (0.25 → 0.5 → 1 → 2 mg). For adults being treated for weight reduction who tolerate 2.4 mg for at least 4 weeks and need further weight reduction, the Wegovy label allows an increase to a maximum of 7.2 mg; the titration calculator's ladder stops at 2.4 mg.

Step	Dose (mg)	Weeks at step	Notes
1	0.25	Weeks 1–4	Starting dose (not a maintenance dose)
2	0.5	Weeks 5–8
3	1	Weeks 9–12
4	1.7	Weeks 13–16
5	2.4	Week 17 onward	Recommended maintenance

Build a personalized version, with your own start dose, target and interval, in the titration calculator.

See the Model in Action

Enter your doses and see what the model predicts. Compare protocols, watch levels accumulate, and see where they level off.

Open Calculator View Dosing Scenarios

Model Limitations

No model predicts an individual's drug levels. The main limits of this one:

Population averages: the parameters are typical values from labels and trials. Real absorption and elimination vary between people and between injections.
One compartment: the population models cited above (the main model in Overgaard 2019, and Schneck 2024) use two compartments. One compartment captures the weekly rise and fall but simplifies the early hours after an injection, and it cannot match both the label's time to peak and the published absorption rate (see above).
Retatrutide inputs are partly assumed: its half-life and time to peak are published, but the time to peak is a Phase 1 median (in the Phase 1b study, blood was sampled mostly 12, 24, 48 and 72 hours after a dose), and its bioavailability is borrowed from tirzepatide. The bioavailability assumption changes the height of the curve only; an error in the time to peak would change where the peak falls and how high it rises.
Amount, not concentration: the chart shows milligrams in the body. The concentration in the Hourly Drug Levels table is a population-average estimate, not adjusted for body weight. Neither predicts your own blood level or can be compared with a lab result.
No dose-response modelling: the calculator shows modelled drug amounts, not clinical effects. Drug levels do not translate directly into weight loss, appetite or side effects.

The purpose of this tool is clarity, not clinical guidance. Seeing modelled levels helps you understand accumulation, steady state, and what happens when you change doses. It does not replace the advice of a doctor who knows your individual health situation.

Technical Implementation

Engine: Python with NumPy. The absorption rate is solved numerically (bisection followed by Newton steps) so the single-dose peak matches the time to peak.
Time resolution: 30-minute steps on the main chart, coarsened only when a very long schedule would exceed 12,000 points; 1-hour steps in the missed-dose, washout and split-dose calculators and on the scenario pages.
Superposition: each time point sums the contributions of all earlier doses, each with its own absorption and elimination.
One model setting: the model is set for the whole site on the server. There is no per-user switch and no "simple mode" in the calculators.
Visualization: Plotly.js renders the interactive charts with hover data, zoom, pan and dose markers.

See Your Levels

Enter your doses and see what the model predicts for your schedule.

Open Calculator

Frequently Asked Questions

What pharmacokinetic model does GLP3Planner use?

A one-compartment model with first-order absorption and first-order elimination (the Bateman function), applied to each dose and added up across the schedule. It plots the modelled amount of drug in the body in milligrams. Each drug's half-life, time to peak and bioavailability set the curve, and the absorption rate is solved from the time to peak. For retatrutide and the injectable forms of semaglutide and tirzepatide (Ozempic and Wegovy injection, Mounjaro, Zepbound), every calculator that draws a drug-level curve uses this model; there is no simple-decay mode to switch to. The calculator cannot model the tablets: a tablet dose entered as semaglutide, Ozempic or Wegovy is treated as a weekly injection and the chart will be wrong, and an entry named Rybelsus falls back to simple half-life decay.

Where does GLP3Planner get its drug parameters?

Semaglutide and tirzepatide values come from section 12.3 of the FDA labels (Wegovy, Ozempic, Mounjaro and Zepbound). Retatrutide's half-life and time to peak (24 hours) come from its Phase 1 studies (Coskun 2022 and Urva 2022). Two values are chosen where no published figure exists, and the parameter table marks them as assumptions: retatrutide's bioavailability (80%, borrowed from tirzepatide) and semaglutide's time to peak (2 days, the midpoint of the label's 1 to 3 days).

Why is retatrutide's bioavailability an assumption?

Because no figure has been published. PubMed and Europe PMC searches and the full texts of the main retatrutide studies, checked on 23 October 2026, turned up no absolute bioavailability value. Lilly's study with an intravenous part (NCT05959096) completed in July 2024 with no results posted. The calculator assumes 80%, the tirzepatide value. It only changes the height of the retatrutide curve, not its shape or timing.

Do the dosing scenario pages use the same model as the calculators?

Yes. They run the same engine with the same parameters, in 1-hour steps, so a scenario's peak, trough and washout figures match what the calculators show for the same schedule. Before 23 October 2026 they used simple half-life decay with no absorption phase, which put the peak of each scenario's last dosing phase about 30% to 41% higher.

How accurate is the GLP3Planner calculator?

It uses population-average parameters, so it shows the typical pattern: how levels build up, roughly when they level off, and how one schedule compares with another. It does not predict your own levels, which depend on your body and your injections, and retatrutide's curve rests partly on assumptions. It is most useful for comparing protocols, not for predicting an exact personal number.

How does the calculator handle multiple doses?

By superposition: at each time point it computes what every earlier dose still contributes and adds those amounts together. Because the half-lives are 5 to 7 days, weekly doses overlap, levels climb over the first weeks, and they level off once the amount removed each week matches the dose.

Why does my chart show increasing levels even at the same dose?

That is accumulation. Each weekly dose is given before the previous one has cleared, so the leftover drug adds to the new dose until steady state. The labels put steady state at 4 weeks for tirzepatide and 4 to 5 weeks for semaglutide (Ozempic); for retatrutide, about 4 weeks is an estimate from its 6-day half-life. In the calculator's model, the level before the 5th weekly dose is already 94% to 98% of its final value.

Can I use this calculator for compounded peptides?

The calculator uses the same parameters whatever the source of the drug. Those parameters come from studies of the manufacturers' products, so the curve only applies if your vial holds the stated amount of the same molecule, which the calculator cannot check. The reconstitution calculator works out syringe units for compounded vials.

Does body weight affect the calculator results?

No. The chart shows the modelled amount of drug in the body in milligrams, which depends on the dose, timing, half-life, time to peak and bioavailability, not on body weight. The estimated concentration in the Hourly Drug Levels table uses one population-average volume per drug, so it does not change with body weight either. Real blood levels differ from person to person.

References

Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metab. 2022;34(9):1234-1247.e9. Results and Table 1 (half-life, time to peak, volume of distribution). PubMed
Urva S, Coskun T, Loh MT, et al. LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial. Lancet. 2022;400(10366):1869-1881. Abstract and supplementary appendix (Table S3, time to peak; protocol, sampling schedule) checked; article full text not reachable. PubMed · Appendix
Eli Lilly and Company. A Study of LY3437943 in Healthy Participants With a High Body Mass Index (NCT05959096). ClinicalTrials.gov. Completed July 2024; no results posted as of 23 October 2026. ClinicalTrials.gov
Novo Nordisk. Wegovy (semaglutide) prescribing information (injection and tablets), sections 2.2 and 12.3. DailyMed, U.S. National Library of Medicine. DailyMed
Novo Nordisk. Ozempic (semaglutide) injection prescribing information, section 12.3. DailyMed, U.S. National Library of Medicine. DailyMed
Eli Lilly and Company. Zepbound (tirzepatide) injection prescribing information, section 12.3. DailyMed, U.S. National Library of Medicine. DailyMed
Eli Lilly and Company. Mounjaro (tirzepatide) injection prescribing information, section 12.3. DailyMed, U.S. National Library of Medicine. DailyMed
Overgaard RV, Delff PH, Petri KCC, Anderson TW, Flint A, Ingwersen SH. Population pharmacokinetics of semaglutide for type 2 diabetes. Diabetes Ther. 2019;10(2):649-662. Tables 4 and 5 (absorption rate constant). PMC
Schneck K, Urva S. Population pharmacokinetics of the GIP/GLP receptor agonist tirzepatide. CPT Pharmacometrics Syst Pharmacol. 2024;13(3):494-503. Table 3 (absorption rate constant). PMC
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Medical Disclaimer

This page is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or recommendations. The pharmacokinetic models described here produce estimates based on population-average data and do not predict individual drug levels. Always consult your doctor before starting, adjusting, or stopping any medication.