Subcutaneous Injection Guide | Investigational (Phase 3)
Retatrutide Injection Sites
Retatrutide (LY3437943) is an investigational triple-agonist GLP-1, GIP, and glucagon receptor agonist injected subcutaneously once a week. It has no FDA-approved label or instructions for use. In Lilly's injection-site study (NCT05959096) it was injected under the skin of the thigh, upper arm, or abdomen. Below you'll find each site explained, how to rotate them, prep steps, and which local reactions are worth watching.
Key Takeaways
- 3 sites used in Lilly's injection-site study: Abdomen, thigh, and upper arm (Lilly's injection-site study, NCT05959096)
- Self-injection: Abdomen and thigh are suitable for self-administration. Upper arm is difficult to reach and control without a caregiver.
- Rotate with every dose: General injection guidance is to use a different area each time
- Avoid damaged skin: Do not inject into tender, bruised, damaged, or scarred skin within any site
- Investigational status: Retatrutide has no FDA-approved label or instructions for use. Site guidance here comes from that study and general subcutaneous injection technique.
- Alcohol prep: Clean the injection site with an alcohol pad before each injection (general technique, MSKCC)
3
Sites in Lilly's Site Study
Once weekly
Dosing Frequency
~6 days
Half-Life (supports weekly schedule)
Investigational compound: Retatrutide (LY3437943) has not received FDA approval as of October 2026. Phase 3 trials are ongoing, and there is no FDA-approved label or instructions for use. The sites in this article are the ones used in Lilly's injection-site study (NCT05959096); the technique is general subcutaneous injection guidance. Always follow your prescriber's instructions.
The 3 Retatrutide Injection Sites in Lilly's Site Study
Retatrutide has no FDA-approved label, so no official document lists its injection sites. In Lilly's injection-site study (NCT05959096), retatrutide was "administered subcutaneously (SC) in either thigh, upper arm, or abdomen". Those are the same three areas the FDA labels of the related approved drugs Zepbound (tirzepatide) and Wegovy (semaglutide) use. Here's what each one looks like and when to use it.
Abdomen (Most Common for Self-Injection)
The abdomen is the most practical site for self-injection. You inject into the subcutaneous fat layer of the stomach area, not into muscle. The usable zone covers the soft tissue on the left and right sides of the navel, below the waistline, and toward the lower abdomen.
Retatrutide has no label to set a distance from the navel. General subcutaneous injection guidance from Memorial Sloan Kettering Cancer Center (MSKCC) says to use the abdomen "except for the 2-inch (5-centimeter) area around your belly button", and to choose a site where you can pinch a 1 to 2 inch fold of skin.
Practical tip: Many people find the lower abdomen, slightly to the left or right, is the most comfortable and accessible area. The skin there tends to have enough subcutaneous fat to allow a clean pinch if needed before inserting the needle.
Thigh (Front and Outer Only)
The front and outer thigh is a reliable alternative to the abdomen. Inject into the subcutaneous tissue of the middle third of the thigh, roughly midway between the knee and hip, on the front or outer side. This site is especially useful when abdominal skin is tender or when you have recently used that area.
Many people find the thigh easier to visualize and access, particularly while seated. Avoid the inner thigh, which has less subcutaneous fat and is more sensitive tissue.
Upper Arm (Caregiver-Assisted in Practice)
Practical limitation: Retatrutide has no FDA-approved label or instructions for use, so nothing official says whether you can inject your own upper arm. The label of the related approved drug Zepbound (tirzepatide) says to inject "in the abdomen, thigh, or another person should inject in the back of the upper arm" (section 2.4), and MSKCC's general guide lists the back of the upper arm only "if someone else is giving you the injection". In practice, the back of the upper arm (tricep area) is difficult to reach and control for self-injection, so most people who use it do so with caregiver assistance. Ask your prescriber whether self-injection into the upper arm is feasible for you given your anatomy and device type.
When the upper arm is used, the usual target is the back of the upper arm (the tricep area), where there is enough fat under the skin. That is the location the Zepbound label and MSKCC's guide both name.
How to Rotate Retatrutide Injection Sites and Why It Matters
Retatrutide has no label to say so, but general subcutaneous injection guidance is clear. MSKCC's guide says: "Give the injection in a different area each time", because injecting in the same spot "will make scar tissue form", which "can keep the medicine from working like it's supposed to". The labels of the related approved drugs say the same: Zepbound's and Wegovy's both say to "Rotate injection sites with each dose." Repeated injection into the same spot can make the fat under the skin thicken and become fibrous, a condition called lipohypertrophy, and absorption from that site becomes erratic.
Because retatrutide has a half-life of approximately 6 days and is dosed once weekly, you return to each site roughly every 4 weeks when following a structured rotation. This 4-week cycle also aligns with the 4-week titration interval, making it straightforward to tie site rotation to your dose escalation calendar.
A Simple 4-Week Retatrutide Rotation Schedule
A consistent rotation pattern makes it easy to remember where you last injected. A 4-week cycle works well for most people on weekly dosing:
Week 1
Left abdomen
Week 2
Right abdomen
Week 3
Left thigh
Week 4
Right thigh
Repeat the cycle from Week 1 after Week 4. If a caregiver assists, the upper arm can substitute for any week in the cycle.
What Happens If You Do Not Rotate? (Lipohypertrophy)
Injecting into the same location every week causes lipohypertrophy: thickened, fibrous subcutaneous fat from repeated tissue trauma. The affected area may feel firm or rubbery under the skin. Beyond the cosmetic concern, lipohypertrophic tissue absorbs medication erratically. People who develop it sometimes see inconsistent weekly responses that have nothing to do with the dose itself.
If you develop a firm raised area at a previous injection site, avoid that spot until it resolves and mention it to your prescriber at the next appointment.
Preparing to Inject Retatrutide: Step-by-Step
Good preparation is most of the job. Retatrutide has no label or instructions for use, so the steps below follow standard subcutaneous injection technique from MSKCC.
What You Need
- Reconstituted retatrutide vial or pen device
- Appropriate insulin syringe if drawing from a vial (see the retatrutide reconstitution guide for mixing steps)
- Alcohol swabs
- Sharps disposal container
- Optional: gauze or cotton ball for post-injection pressure
Site Prep Steps
- Choose your site for this week from one of the 3 sites, rotating from last week's location
- Clean with an alcohol pad and let the skin air dry before injecting (MSKCC: "Let your skin air dry. Don't fan or blow on the area.")
- Pinch the skin gently if adequate subcutaneous fat is present, per MSKCC subcutaneous injection guidance
- Insert the needle at a 45 to 90 degree angle depending on needle length and tissue depth (shorter needles use 90 degrees; longer needles on leaner tissue use 45 degrees)
- Inject the dose slowly, withdraw the needle, and apply gentle pressure with gauze. Do not rub the site after withdrawing.
- Dispose of the needle immediately in a sharps container. Never recap used needles.
Does the Injection Site Affect How Well Retatrutide Works?
Retatrutide has no FDA label, so there is no labelled ranking of injection sites. For the related drugs, the labels report that site made little difference to exposure: "Similar exposure was achieved with subcutaneous administration" of tirzepatide (Zepbound) and of semaglutide (Wegovy) "in the abdomen, thigh, or upper arm". No comparable result has been posted for retatrutide.
No absolute bioavailability figure for retatrutide has been published. The first-in-human study (Coskun et al., 2022) reports the half-life and time to peak but not bioavailability, and a search of PubMed and Europe PMC on 23 October 2026 found no paper that does. So there is no retatrutide figure for how much of a dose is lost at the injection site, at any location. Lilly has run the study that would answer the site question: NCT05959096 compared absorption from the upper arm and thigh with the abdomen, and gave an intravenous dose in a second part, but no results had been posted on ClinicalTrials.gov as of 23 October 2026. Until they are, claims about meaningful absorption differences between the abdomen, thigh, and upper arm have no retatrutide-specific Tier 1 evidence behind them.
The Pharmacokinetic Case for Rotating on Schedule
With a ~6-day half-life, retatrutide levels stay detectable all week and reach steady state after about 4 weeks of consistent dosing (estimated from the ~6-day half-life). Rotating sites keeps each dose absorbing from healthy, intact tissue. Lipohypertrophic spots absorb erratically and can flatten or spike the curve in ways that have nothing to do with your dose.
The GLP3Planner retatrutide calculator lets you visualize that drug level curve week to week, so you can see what consistent timing and proper rotation actually do to steady-state exposure.
Retatrutide Injection Site Reactions: What Is Normal, What Is Not
Minor local reactions are common with subcutaneous injections. Clinical data for retatrutide shows injection site reactions in approximately 5 to 15 percent of participants, typically mild redness, itching, or small lumps that resolve within a day or two.
Usually normal
- Small red bump or raised area at the site
- Mild itching that fades within hours
- Minor bruising from the needle
- Brief stinging or pinching during injection
Contact your provider
- Spreading redness or warmth around the site
- Pus or discharge from the injection point
- Swelling that worsens after 24 hours
- Signs of allergic reaction: hives, swelling away from site, difficulty breathing
Rotating sites each week and using correct technique reduce local reaction frequency. For a full breakdown of retatrutide adverse events beyond the injection site, see the retatrutide side effects guide.
Track Your Retatrutide Injection Schedule
Log each weekly dose and site with the free GLP3Planner calculator. Visualize retatrutide drug levels and stay on your titration schedule.
How to Inject Retatrutide: Video Demonstration
This demonstration by board-certified dermatologist Dr. Jason Emer, MD covers the full subcutaneous injection process including site selection, needle angle, and preparation steps. Subcutaneous injection technique is consistent across GLP-1 class drugs, making this 464,000-view walkthrough directly applicable to retatrutide pen and vial administration.
Dr. Jason Emer, MD. "HOW TO PREPARE AND INJECT PEPTIDES WITH DEMONSTRATION OF ALL STEPS | SEMAGLUTIDE." Published April 2024. 464,193 views. Subcutaneous injection technique shown is applicable across GLP-1 class drugs including retatrutide.
Retatrutide Injection Sites: Frequently Asked Questions
Where should I inject retatrutide?
Retatrutide has no FDA-approved label, so there is no official list of sites. In Lilly's injection-site study (NCT05959096) it was injected under the skin of the thigh, upper arm, or abdomen. The abdomen and thigh are the practical choices for self-injection. General injection guidance is to use a different area each time and to clean the skin with an alcohol pad first.
How often should retatrutide be injected?
In its trials, retatrutide was injected under the skin once a week. Its half-life of approximately 6 days supports this schedule. Drug levels remain detectable throughout the week, and steady state takes about 4 weeks, an estimate from the half-life.
Can I self-inject retatrutide into my upper arm?
Retatrutide has no FDA-approved label, so nothing official says. The upper arm was one of the three sites in its injection-site study, but it is difficult to reach and control for self-injection, and MSKCC's general guide lists the back of the upper arm only if someone else gives the injection. In practice, the abdomen and thigh are the two sites most suitable for self-administration.
What happens if you do not rotate retatrutide injection sites?
Repeated injection into the same spot causes lipohypertrophy: thickened, fibrous subcutaneous fat that absorbs medication erratically. General injection guidance is to use a different area each time. If you develop a firm raised area at a previous injection site, avoid it and inform your prescriber at the next appointment.
Does the injection site affect how well retatrutide works?
Retatrutide has no FDA label, so there is no labelled ranking of injection sites. For tirzepatide and semaglutide, the FDA labels report similar exposure from the abdomen, thigh, or upper arm. No bioavailability figure for retatrutide has been published (PubMed and Europe PMC searched 23 October 2026), and Lilly's injection-site study (NCT05959096) had posted no results by that date, so there is no data on whether the share of a dose that is absorbed differs between sites.
What skin should I avoid when injecting retatrutide?
Avoid skin that is tender, bruised, damaged, or scarred when choosing your injection spot within a site. These conditions impair absorption and raise the risk of discomfort or local infection. Choose clean, intact subcutaneous tissue within the abdomen, thigh, or upper arm. Avoid the navel zone as standard subcutaneous technique.
What are common retatrutide injection site reactions?
Injection site reactions occur in approximately 5 to 15 percent of people using retatrutide in clinical data. Common reactions are mild redness, itching, or small lumps that resolve within a day or two. Rotating sites each week, choosing healthy skin, and using correct technique reduce the frequency of local reactions.
Why should you not inject near the belly button?
The tissue around the navel is denser than the surrounding subcutaneous fat, which can make the injection more uncomfortable and affect how evenly medication absorbs. Standard subcutaneous injection technique recommends avoiding this zone. Retatrutide has no label to set a distance; MSKCC's general guide says to stay 2 inches (5 cm) from the belly button.
How do you inject retatrutide from a vial?
Injecting from a retatrutide vial requires reconstitution with bacteriostatic water first. Draw up the calculated dose into an insulin syringe, select one of the three sites, clean with alcohol, pinch skin gently, inject at a 45 to 90 degree angle, and dispose of the needle in a sharps container. See the reconstitution guide for full mixing steps.
What time of day should I inject retatrutide?
Retatrutide has no FDA-approved label, so there is no official instruction on time of day. In its trials it was injected once a week. Keeping the same day each week matters more than which time you choose. Using the GLP3Planner calculator to log your injection day helps maintain a consistent weekly schedule as you titrate through the dose steps.
References
- Jastreboff AM, Kaplan LM, Frias JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. New England Journal of Medicine. 2023;389(6):514-526. doi:10.1056/NEJMoa2301972
- Memorial Sloan Kettering Cancer Center. How to Give Yourself a Subcutaneous Injection Using a Prefilled Syringe. Available at: mskcc.org Source for general subcutaneous injection technique: site areas, the 2-inch navel clearance, rotating sites, alcohol prep.
- Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple GIP, GLP-1 and glucagon receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metabolism. 2022;34(9):1234-1247.e9. doi:10.1016/j.cmet.2022.07.013 Source for the approximately 6-day half-life and the once-weekly dosing pharmacokinetic rationale. It reports no bioavailability figure.
- Eli Lilly and Company. A Study of LY3437943 in Healthy Participants With a High Body Mass Index (NCT05959096). ClinicalTrials.gov. NCT05959096 Source for the three injection sites: Part A gave retatrutide "subcutaneously (SC) in either thigh, upper arm, or abdomen". Compares absorption from the upper arm and thigh with the abdomen, and includes an intravenous part. Completed July 2024; no results posted as of 23 October 2026.
- Eli Lilly and Company. Zepbound (tirzepatide) injection, prescribing information, sections 2.4 and 12.3. DailyMed. dailymed.nlm.nih.gov Label of a related approved drug: sites, rotation, and similar exposure across sites. Not a retatrutide source.
- Novo Nordisk. Wegovy (semaglutide) prescribing information, sections 2.1 and 12.3. DailyMed. dailymed.nlm.nih.gov Label of a related approved drug: rotation, and similar exposure across abdomen, thigh, and upper arm. Not a retatrutide source.
Medical Disclaimer
This article is for educational purposes only and does not constitute medical advice. Retatrutide (LY3437943) is an investigational compound that has not been approved by the FDA as of October 2026. Phase 3 trials are ongoing and this information may change. Do not start, stop, or change your dose or injection technique without guidance from a licensed clinician. Retatrutide has no FDA-approved label or instructions for use; the sites here are the ones used in Lilly's injection-site study (NCT05959096), and the technique is from standard subcutaneous injection references. Always defer to your prescriber. Results from clinical trials represent group averages and individual results vary.
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