The basics: most side effects from these medications are digestive, they show up in the first weeks and again after each dose increase, and most of them ease as your body adjusts. A few specific symptoms mean you should call your doctor instead of waiting it out. The two things that help most are how you manage your doses and how you eat.
What should you expect, and when should you call a doctor?
Digestive upset is the common thread. Mostly nausea, diarrhea, vomiting and constipation, alongside burping, tiredness and headaches. These cluster in the first weeks and again in the days after each dose increase, and for most people they settle as the body adjusts.
How common each one is, and how often the same thing happened on placebo. These figures come from the trials behind Wegovy’s approval. Wegovy is the 2.4 mg dose of semaglutide. The trials compared 2,116 people on the drug against 1,261 on placebo.
| Side effect | On the medication | On placebo |
|---|---|---|
| Nausea | 44% | 16% |
| Diarrhea | 30% | 16% |
| Vomiting | 25% | 6% |
| Constipation | 24% | 11% |
| Headache | 14% | 10% |
| Tiredness | 11% | 5% |
| Burping | 7% | 0.4% |
| Hair loss | 3% | 1.4% |
| Reactions at the injection site | 1.4% | 1.0% |
The placebo column is worth reading carefully. A good number of people report these symptoms with no medication in them at all. So the drug’s real contribution is the gap between the two columns, not the headline number. For example, in one study, while 44% of people receiving semaglutide experienced nausea, 16% of people receiving a placebo (no semaglutide) also experienced nausea. And while many people worry about reactions at the injection site, the semaglutide reaction (1.4% of users) was barely above placebo (1.0%).
These figures are from the Wegovy trials specifically. Rates differ by medication and by dose, and are generally lower on the diabetes-dose products. Your medication’s own page carries its numbers: Wegovy, Zepbound, Mounjaro, Ozempic, or Rybelsus.
When to call your doctor instead
Most of this you can manage at home. These you cannot. Call your prescriber if you have:
- Vomiting that is severe, or that will not stop
- Trouble keeping fluids down
- Severe pain in your belly, especially pain that reaches through to your back
- Pain in the upper right of your belly, particularly after fatty food
- A swollen belly when you have stopped passing gas or stool
Our GLP-1 safety page carries the full warnings and safety information.
What helps alleviate symptoms in general?
Two things do more than everything else combined: how fast you move up doses, and how you eat.
Dose pace matters most. Every GLP-1 climbs a dose ladder, and the ladder exists to give your gut time to adjust. If a dose increase leaves you unwell, holding at your current dose for longer is a normal adjustment. Prescribers recommend it often. It is not a setback.
So tell your prescriber if side effects are getting hard to live with. Slowing down is something they can do. Stopping is not your only option.
The eating changes are the ones you have probably already read about:
- Eat smaller portions than you think you need
- Stop at the first sign of fullness rather than finishing what is in front of you
- Eat slowly, because fullness registers late and it is easy to overshoot
- Ease off very fatty and spicy food while your body is adjusting
- Keep fluids up, particularly if constipation is your problem
What you eat matters more once you are eating less of it, which is its own question about what to eat.
These come from what clinicians consistently see rather than from a trial that tested them head to head, and the research on how much dose timing changes tolerability is thin. We grade the evidence on side effects separately, including where it runs out. They are what prescribers consistently recommend, and they cost nothing to try. If you are just starting, what to expect in the first months covers how this fits into the wider picture, including what to set up early.
How do you handle the digestive symptoms?
These five account for most of what people feel, and they behave differently from each other. Nausea front-loads and fades. Constipation often arrives later, once the early queasiness has settled. Diarrhea catches people out entirely, because the constipation reputation suggests the opposite.
Nausea
The most common by a wide margin, and usually worst in the first weeks and in the days after a dose increase. It tends to be a background queasiness, not the acute kind, and it often arrives alongside getting full much sooner than you expect. Many people describe it as feeling permanently slightly car-sick, without the distinct episodes of feeling ill.
Smaller portions and eating slowly help most here, because the mechanism behind the nausea is the same one behind the fullness: food is leaving your stomach more slowly than you are used to. Eating to the volume your stomach can now handle, not the volume you are used to, is most of the fix.
If nausea has not eased by the time you have been at a dose for a few weeks, that is worth raising before the next increase rather than after it. If it is tipping into vomiting you cannot control, that belongs in the call list above.
Vomiting
Less common than nausea but reported by about a quarter of people in the trials, and most likely in the days after a dose step. Occasional vomiting in that window is within the ordinary range.
Vomiting that is severe, will not stop, or is keeping you from holding down fluids is not, and that is a call to your prescriber rather than something to push through. It matters more than the discomfort suggests, because of dehydration. The kidney problems occasionally reported with these medications are usually traced to fluid loss from vomiting and diarrhea, not to the drug harming the kidney itself. Keeping fluids down is the thing to watch.
Constipation
Slowed digestion is part of how these medications work, so slower bowels are an expected consequence, not a sign something has gone wrong. How GLP-1s actually work goes through the mechanism. It also tends to arrive later than nausea does, often once the early queasiness has settled, which catches people out.
Fluids, fibre and movement are what people commonly reach for, and they are reasonable things to try. We have not graded the evidence for fibre supplements, stool softeners or laxatives in people on these medications, so we are not going to tell you they work. What we can say is that eating substantially less makes it harder to get enough fibre by accident, so it is worth being deliberate about it in a way you may not have needed to be before. If it is persistent, your prescriber can advise on what is appropriate for you, and it is worth raising early instead of living with it.
Diarrhea
Diarrhea is the second most common side effect of these medications, reported by about 30% of people on semaglutide against 16% on placebo. The constipation reputation suggests the opposite, and some people alternate between the two instead of getting one or the other.
Keeping fluids up matters more here than anywhere else on this page, for the same dehydration reason as vomiting. Easing off very fatty food tends to help, and if it persists beyond the days following a dose increase it is worth mentioning to your prescriber.
Burping
Burping is reported by about 7% of people on semaglutide against 0.4% on placebo, the largest relative jump of any side effect on the label’s list. It is uncommon without the medication and reasonably common with it, and it is what people are describing when they mention sulfur burps as a side effect.
We can tell you how often it happens. We cannot tell you why some people describe it as sulfurous. The trials recorded that people burped, not what it smelled like, and no good study has looked at the difference.
What about tiredness, headaches, hair loss, and reactions where you inject?
All of these are reported, and all of them look smaller once you read the placebo column.
Tiredness was reported by about one in ten people, against about one in twenty on placebo. The label’s figure combines tiredness and weakness into one category, so it covers a broader range of feeling than the word suggests.
Headache is similar: real, and a smaller gap over placebo than the raw number implies.
Hair loss was reported by about 3% of people on semaglutide against 1.4% on placebo. The trials recorded that it happened without establishing why, and we have nothing graded on whether it is the medication or the weight loss itself doing it. If it is happening to you, that is worth raising with your prescriber instead of assuming either answer.
Reactions where you inject, meaning itching, redness or irritation at the site, were reported by 1.4% of people on semaglutide and 1.0% on placebo, and the FDA reviewer described them as self-limited and occurring at low rates. That gap is small enough that this is not one of the things worth worrying about before you start. How to inject a GLP-1 covers technique, which matters more for comfort than anything else here.
Which side effects are serious, and how likely are they?
These are uncommon, and none of them is something to diagnose yourself. They are here so you can recognize the symptoms and know what to raise, not as a reason to avoid the medication.
Gallbladder problems are an uncommon but real effect across this class of medication. The symptom to know is upper right abdominal pain, especially after fatty food.
Pancreatitis is monitored rather than established. Regulators watch for it, and it appears in the labels, but a causal link has not been shown. The symptom is severe abdominal pain that reaches through to the back.
Slowed stomach emptying is part of how the medication works, and in a small number of people it becomes pronounced enough to cause problems. It is also the reason these medications are worth mentioning to anyone planning a procedure for you.
Bowel obstruction has been reported to the FDA’s adverse-event system. Those reports do not establish that the medication caused them, and the system has no denominator, so no rate can be calculated from it. It is in this list because the symptoms are worth recognizing, not because the risk is quantified.
A thyroid tumour warning appears on every one of these medications. It comes from studies in rats, and no human cases appeared in the trials. It is still a firm contraindication if you or a close family member has had medullary thyroid cancer or MEN-2.
An eye complication called diabetic retinopathy has a signal specific to semaglutide, in people who already have it. This one does not carry across to tirzepatide.
One this page does not cover: the facial volume loss people call Ozempic face, which is fat loss rather than a separate effect of the drug. We go through it separately.
The one that is rarely called a side effect at all: during fast weight loss, some of what comes off is muscle and bone density rather than fat. It is not something you will feel week to week, which is why it goes unmentioned, and it is the one on this list you can most directly influence. What GLP-1s do to your muscle and our evidence on muscle go through it properly.
Sources and review
How common each side effect is, and safety labeling
- US FDA, Center for Drug Evaluation and Research. Summary Review, NDA 215256, Wegovy (semaglutide 2.4 mg injection). Approved June 4, 2021. accessdata.fda.gov
- US FDA, Center for Drug Evaluation and Research. Summary Review, NDA 209637, Ozempic (semaglutide). Approved December 5, 2017. accessdata.fda.gov
- US FDA, Center for Drug Evaluation and Research. Summary Review, NDA 215866, Mounjaro (tirzepatide). Approved May 13, 2022. accessdata.fda.gov
Timing, and why people stop
- Klein KR, Clemmensen KKB, Fong E, Olsen S, Abrahamsen T, Lingvay I. Occurrence of Gastrointestinal Adverse Events Upon GLP-1 Receptor Agonist Initiation With Concomitant Metformin Use: A Post Hoc Analysis of LEADER, STEP 2, SUSTAIN-6, and PIONEER 6. Diabetes Care. 2024;47(2):280-284. doi:10.2337/dc23-1791
Managing the digestive effects
- Wharton S, Davies M, Dicker D, et al. Managing the gastrointestinal side effects of GLP-1 receptor agonists in obesity: recommendations for clinical practice. Postgrad Med. 2022;134(1):14-19. doi:10.1080/00325481.2021.2002616
- Pieber TR, Bode B, Mertens A, et al. Efficacy and safety of oral semaglutide with flexible dose adjustment versus sitagliptin in type 2 diabetes (PIONEER 7): a multicentre, open-label, randomised, phase 3a trial. Lancet Diabetes Endocrinol. 2019;7(7):528-539. doi:10.1016/s2213-8587(19)30194-9
The serious and rare effects
- Filippatos TD, Panagiotopoulou TV, Elisaf MS. Adverse Effects of GLP-1 Receptor Agonists. Rev Diabet Stud. 2014;11(3-4):202-230. doi:10.1900/rds.2014.11.202
Muscle and bone during weight loss
- Eglseer D, Traxler M, Embacher S, et al. Nutrition and Exercise Interventions to Improve Body Composition for Persons with Overweight or Obesity Near Retirement Age: A Systematic Review and Network Meta-Analysis of Randomized Controlled Trials. Adv Nutr. 2023;14(3):516-538. doi:10.1016/j.advnut.2023.04.001
- Sardeli AV, Komatsu TR, Mori MA, Gáspari AF, Chacon-Mikahil MPT. Resistance Training Prevents Muscle Loss Induced by Caloric Restriction in Obese Elderly Individuals: A Systematic Review and Meta-Analysis. Nutrients. 2018;10(4):423. doi:10.3390/nu10040423
Not yet medically reviewed. This page is in editorial review. A named clinical reviewer and review date are required before it is indexed.