How GLP-1 medications actually work
GLP-1 (glucagon-like peptide-1) is a naturally occurring incretin hormone released by the gut after eating. It signals satiety to the brain, slows gastric emptying, and amplifies insulin release in response to glucose. The synthetic GLP-1 receptor agonists used for weight loss, semaglutide, tirzepatide, liraglutide, bind those same receptors with much longer half-lives, sustaining the satiety signal for days rather than minutes.
Tirzepatide adds a second mechanism: it activates the GIP (glucose-dependent insulinotropic polypeptide) receptor as well as GLP-1. The dual mechanism produces stronger appetite suppression and metabolic effects, which is why head-to-head trials show roughly 50% greater weight loss on tirzepatide vs semaglutide.
Semaglutide vs tirzepatide: what the data says
The clean head-to-head was SURMOUNT-2 (2023, NEJM), comparing tirzepatide and semaglutide directly in adults with obesity and type 2 diabetes. Tirzepatide produced approximately 50% greater weight loss over 72 weeks. Side-effect profiles were similar, both cause GI symptoms (nausea, constipation, fatigue) primarily during dose escalation, which resolve in most patients on a stable dose.
That said, semaglutide is the right starting point for many patients: lower starting price (~$249/mo cash-pay vs ~$349/mo for tirzepatide on OPTML), longer real-world data set (Ozempic launched in 2017; Mounjaro/Zepbound in 2022), and easier titration if you're side-effect sensitive.
Most patients we work with start on semaglutide and switch to tirzepatide if they plateau before reaching their goal, or start on tirzepatide directly if speed matters and budget allows.
What to expect: week-by-week timeline
Week 1: Hunger drops within 2-7 days. Mild nausea possible. No measurable weight loss yet.
Weeks 2-4: Scale starts moving, 1-4 lb in the first month is typical at the 0.25 mg semaglutide or 2.5 mg tirzepatide starting dose.
Weeks 5-16: Standard dose escalation every 4 weeks. Each step deepens appetite suppression and unlocks another phase of weight loss.
Weeks 16-24: Steady state on maintenance dose. Most patients are down 8-15% of body weight by month 6.
Weeks 24-72: Long arc. SURMOUNT-1 patients averaged 20.9% loss at 72 weeks on tirzepatide 15 mg; STEP-1 patients averaged 14.9% at 68 weeks on semaglutide 2.4 mg. Real-world adherent patients land in the same range.
Side effects, safety, and what's overhyped
The most common side effects are GI: nausea, constipation, diarrhea, fatigue. Severity peaks 1-3 days after each dose escalation, then attenuates as the body adjusts. Hydration matters more than people expect, under-drinking amplifies every GI symptom.
Rare but serious risks include pancreatitis, gallbladder issues, and (theoretically, from rodent studies) thyroid C-cell tumors. Real-world incidence of these has been low across millions of prescription years for both drugs. A licensed provider screens contraindications during intake.
Overhyped: "GLP-1 face," muscle loss, rebound. Facial fat depletion is real but cosmetic; muscle loss is preventable with adequate protein (0.7-1.0 g/lb of goal weight) and resistance training; rebound is real if you stop without a maintenance plan but largely preventable with a low-dose maintenance protocol.
How to choose your protocol
Use OPTML's Find My Protocol quiz for a personalized recommendation. The general decision tree:
- New to GLP-1s, want to start gentle: semaglutide
- New to GLP-1s, serious weight-loss goal, willing to spend more for stronger effect: tirzepatide
- Plateaued on semaglutide: switch to tirzepatide
- Doing fine on semaglutide: stay; optimize dose with provider; add protein + training
Primary sources cited
- Wilding JPH et al. NEJM 2021;384:989-1002 (STEP-1)
- Jastreboff AM et al. NEJM 2022;387:205-216 (SURMOUNT-1)
- Frías JP et al. NEJM 2021;385:503-515 (SURPASS-2)