GLP-1 & Retatrutide (GLP-3) Calculator & Dosage Chart

How GLP-1 Half-Life Calculations Work

Understanding medication half-life helps you predict drug levels, optimize dosing timing, and know when you'll reach steady state. For retatrutide and the injectable forms of tirzepatide and semaglutide, our calculator models each injection's absorption and elimination from the drug's half-life and time to peak.

This matters because titrating up gets tricky with half-lives this long. It's easy to lose track of how much is actually in your system, especially when dosing multiple times per week. For example, in GLP3 Planner's model a 2mg dose of Retatrutide peaks at about 1.43mg in your body about a day after the injection, because not all of a dose is absorbed and some is already being cleared. If you don't feel much and take 3mg the following week, about 0.74mg of the first dose is still there when you inject, so the modelled amount in your body peaks at about 2.8mg, compared with about 2.1mg from the 3mg dose on its own. These are modelled amounts, not blood levels, and for Retatrutide the bioavailability is the model's assumption. This compounds week after week. GLP3 Planner helps you visualize the total amount in your system and track how you feel, so you can make informed titration decisions.

Steady-state is typically reached after 4-5 half-lives of consistent dosing. This is when peak and trough concentrations stabilize at predictable levels.