Semaglutide vs Tirzepatide vs Retatrutide: Half-Life, Dosing Cadence, and What to Track

Semaglutide and tirzepatide are the two everyone compares. Retatrutide is the one I want to talk about most, and I saved that part for the end so the plain pharmacology comes first.

What semaglutide, tirzepatide, and retatrutide actually are

Semaglutide is a GLP-1 receptor agonist: it activates the same receptor as the naturally occurring GLP-1 hormone, which affects appetite, gastric emptying, and insulin response. Tirzepatide is a dual agonist: it activates both the GIP and GLP-1 receptors. Retatrutide goes one further (a triple agonist targeting the GIP, GLP-1, and glucagon receptors), but unlike the other two it is investigational: it is still in clinical trials and is not approved by the FDA for any use as of this writing, so it is included here for pharmacological comparison only. Semaglutide and tirzepatide are prescription injectable medications, typically self-administered once a week, and are marketed under various brand names depending on the indication (diabetes vs. weight management). This post uses the generic compound names throughout.

Half-life, in the numbers

The chart below plots a single dose of each compound decaying on its own, using the approximate half-lives from published pharmacokinetic data. It isolates one variable: how fast each one clears on its own, separate from the titration schedule or your own response.

Why the half-life difference matters for tracking, not for “which is better”

Both half-lives are long enough to support a once-weekly schedule without a meaningful trough dip for most people. The difference between roughly 5 and 7 days is real but modest. Individual response, titration pace, and side-effect tolerance move outcomes far more than this pharmacokinetic gap does. Treat the half-life numbers as background for understanding the schedule, not as a reason to pick one over the other.

What actually drives side effects on either one

The half-life gap between the two is small enough that it is rarely what people notice day to day. Titration is what people notice. Both medications start at a low dose and step up on a schedule set by the prescriber, and GI symptoms (nausea, reduced appetite, occasional constipation or diarrhea) tend to cluster right around a step-up rather than building steadily over the whole week. That pattern is the same on either compound, which is part of why it is treated as a titration effect rather than a property of one medication specifically.

If a titration step is hitting harder than expected, the standard response is holding at the current dose for longer before stepping up again, rather than pushing through or skipping doses, but that is a call for your prescriber to make, not something to decide from a blog post. What is useful for you to bring to that conversation is a clean log: which dose you were on, when it changed, and what symptoms showed up and when. That is the exact kind of pattern a symptom-and-dose log makes obvious and memory alone does not.

What to actually track week to week

Regardless of which one you are on, the same handful of things make the log useful later: the dose and titration step for that week, your weight trend (not any single reading), GI symptoms if they show up, and injection site rotation. If labs are part of your monitoring (A1c, lipids, or others your physician orders), logging the protocol you were on at the time each result was drawn is what makes the trend interpretable months later — the same timing discipline that matters for tracking TRT bloodwork applies here too.

The one that sold me

Retatrutide was the one that flipped me all-in on this whole thing. It did what it said it would. What sold me wasn’t only the appetite suppression; it was the increased insulin sensitivity, which also helped push back against HGH’s tendency the other way. I ran it while cutting weight, alongside HGH, to lose fat without losing muscle.

Retatrutide is investigational and not FDA-approved for anything as of 2026. In trials it produced strong appetite suppression and weight loss and improved blood-sugar control, with the insulin-sensitivity gains appearing largely tied to that weight loss. Anything sold as retatrutide outside a trial is unregulated research-grade material of unknown purity.

Whichever one you land on, tracking it is the same problem: dose, titration step, weight trend, and how it interacts with anything else you’re running. That’s part of why phased dosing with automatic step-up lives inside Stackeddd instead of staying a note to self.

Frequently asked questions

Sources

• Semaglutide, terminal half-life about 1 week. Ozempic prescribing information. FDA.
• Tirzepatide, about 5 days. Both molecules buy that duration the same way, by binding reversibly to serum albumin through a fatty-acid side chain and resisting DPP-4, which is the same trick discussed on the CJC-1295 page in a different context. Population pharmacokinetics of tirzepatide, CPT: Pharmacometrics & Systems Pharmacology, 2024.
• A consequence of that. Tirzepatide's half-life is shorter than its dosing interval, so it accumulates from injection to injection and does not reach steady state for roughly 4 to 5 weeks. Judging how a dose is working before then means judging an incomplete picture.
• Retatrutide. The roughly 6-day half-life is from published phase-2 work. It remains investigational and is not approved for any use, so it appears here for pharmacological comparison only.
• Not claimed here: that either drug is better. Head-to-head weight-loss comparisons exist but differ in population, dose and duration, and none of that is what this page is about, which is why the two are dosed the way they are.

This is educational information, not medical advice. Semaglutide and tirzepatide are prescription medications and retatrutide is an investigational compound not approved for use. Decisions about starting, switching, or dosing belong with your prescribing physician.