GLP-1 medicines, explained simply
What semaglutide and tirzepatide do, how fast they work, what trials found, and the side effects to know about.
What GLP-1 is
GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It tells your brain you've had enough, slows how fast your stomach empties, and helps your body release insulin when blood sugar rises.
GLP-1 medicines are lab-made versions that last much longer than the natural hormone, so one injection works for a whole week. For many people the effect feels simple: smaller portions feel like enough, cravings get quieter, and food stops taking up so much space in their head.
Semaglutide and tirzepatide
Two medicines in this family are approved for chronic weight management in the United States. Both are once-weekly injections given with a small pen just under the skin of the belly, thigh or upper arm.
| Feature | Semaglutide | Tirzepatide |
|---|---|---|
| How it works | Acts on GLP-1 receptors | Acts on GIP and GLP-1 receptors |
| Brand names | Wegovy (weight), Ozempic (diabetes) | Zepbound (weight), Mounjaro (diabetes) |
| How often | Once a week | Once a week |
| Starting dose | 0.25 mg, for 4 weeks | 2.5 mg, for 4 weeks |
| Usual dose steps | 0.5, 1, 1.7, then 2.4 mg | Up by 2.5 mg at a time, to 5, 10 or 15 mg |
Your clinician chooses the medicine and the schedule. Some people stay on a lower dose because it works for them or because side effects make a higher dose uncomfortable, and that's fine.
What to expect, and how fast
Doses start low and step up about every four weeks. This slow climb is deliberate: it gives your digestive system time to adjust and keeps side effects milder.
- Weeks 1 to 4. Many people notice appetite changes within days. Weight change is often modest at this stage.
- Months 2 to 4. As the dose rises, appetite effects usually get stronger. This is when your clinician checks in most closely.
- Months 4 to 12 and beyond. In trials, weight loss kept building for well over a year before levelling off.
In the STEP 1 trial, adults taking semaglutide 2.4 mg alongside diet and activity changes lost about 15% of their body weight on average over 68 weeks, compared with about 2.4% on placebo. In SURMOUNT-1, adults on the highest tirzepatide dose lost about 21% on average over 72 weeks. These are averages: some people lost much more, some much less, and a minority lost little. Results vary.
What about stopping? GLP-1 medicines work while you take them. In a follow-up of STEP 1, people who stopped regained much of the weight they had lost within a year. That's why we plan for the long term from day one, including habits that help whatever you decide about medication later.
Side effects
Most side effects are digestive and are usually mild to moderate. They tend to be strongest in the days after a dose increase and ease as your body adjusts.
- Nausea, sometimes vomiting
- Constipation or diarrhea
- Heartburn, burping or bloating
- Tiredness, headache, or feeling full very quickly
Rarer but serious side effects include pancreatitis (severe stomach pain that may spread to the back), gallbladder problems, and kidney strain from dehydration if you can't keep fluids down. If you get severe stomach pain, persistent vomiting, or signs of an allergic reaction, contact your clinician straight away or seek urgent care.
Who should not take them
GLP-1 medicines are not right for everyone. Your clinician will go through your history carefully. They are generally not prescribed if you:
- Have a personal or family history of medullary thyroid carcinoma, or multiple endocrine neoplasia type 2 (MEN 2)
- Are pregnant, trying to become pregnant, or breastfeeding
- Have had a serious allergic reaction to the medicine
Extra care is needed if you have had pancreatitis, have severe stomach or bowel problems such as gastroparesis, have diabetic eye disease, or take insulin or a sulfonylurea (because of the risk of low blood sugar). Tell your clinician about every medicine and supplement you take.
Questions to bring to your consultation
- Based on my history, which medicine, if any, would you suggest, and why?
- What labs do I need before I start?
- What should I do if I feel sick after a dose?
- How will we decide when to increase, hold or lower my dose?
- What does the long-term plan look like?
Educational only, not medical advice; a licensed clinician decides eligibility after your consultation. If you have severe symptoms, contact your clinician or call 911.