GLP-1 weight-loss timeline: what to expect month by month
A realistic month-by-month guide to GLP-1 treatment: dose increases, when appetite changes, how weight loss builds and what happens after a year.
Tirzepatide and semaglutide are the two GLP-1 medicines most people ask about. Both are once-weekly injections, both reduce appetite, and both have large clinical trials behind them. But they aren't identical, and the differences matter when you and your clinician choose a plan.
The short answer
- Tirzepatide acts on two hormone receptors (GLP-1 and GIP). Semaglutide acts on one (GLP-1).
- In a 72-week head-to-head trial of the brand-name medicines, people on tirzepatide lost more weight on average: 20.2% vs 13.7%.
- Semaglutide has the longer track record, including a large heart-outcomes trial, and usually costs less.
- Side effects are similar for both: mostly nausea, diarrhea and constipation, especially when the dose goes up.
How each one works
After you eat, your gut releases hormones called incretins. GLP-1 tells your brain you're full, slows how quickly your stomach empties and helps your pancreas release insulin. GIP is a second incretin that also affects insulin and appears to influence appetite and how the body handles fat.
Semaglutide is a long-acting version of GLP-1. Tirzepatide is a single molecule that activates both GLP-1 and GIP receptors. That dual action is thought to be one reason it produced more weight loss in trials.
What the research shows
| Semaglutide 2.4 mg | Tirzepatide 15 mg | |
|---|---|---|
| Pivotal trial | STEP 1 (68 weeks) | SURMOUNT-1 (72 weeks) |
| Average weight loss | 14.9% (placebo 2.4%) | 20.9% (placebo 3.1%) |
| Head-to-head (SURMOUNT-5) | 13.7% | 20.2% |
| Heart outcomes | 20% fewer major cardiac events (SELECT) | Trial ongoing |
These numbers come from trials of the FDA-approved, brand-name medicines, used together with diet and activity advice. Compounded versions were not studied in these trials, and individual results vary widely: some people lose much more than the average, some much less.
Side effects
Both medicines most often cause gastrointestinal side effects: nausea, diarrhea, constipation, vomiting and indigestion. In STEP 1, 44% of people on semaglutide reported nausea at some point, compared with 17% on placebo. In SURMOUNT-1, nausea affected about 25–33% of people depending on dose, compared with about 10% on placebo. These effects are usually mild to moderate and happen most when starting or increasing a dose.
Both carry the same boxed warning about thyroid C-cell tumors seen in rodents, and the same cautions about pancreatitis, gallbladder disease, low blood sugar with certain diabetes medicines, and dehydration.
Which should you choose?
There isn't a universal best choice. Your clinician will consider your goals, your health history, how you've responded to medication before, your budget and any side effects you're worried about.
- Tirzepatide may suit you if you want the strongest average effect seen in trials and the price difference works for you.
- Semaglutide may suit you if cost matters most, or if you'd prefer the medicine with the longest safety record.
- You can switch later. If one isn't working or isn't tolerated, your clinician can discuss changing.
Sources
- Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022;387:205-216.
- Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med. 2021;384:989-1002.
- Aronne LJ et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med. 2025. Trial record NCT05822830.
- Lincoff AM et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). N Engl J Med. 2023;389:2221-2232.
This article is for general information and isn’t medical advice. See our Important Safety Information.
