GLP-1 side effects, from the labels
Bottom line
constipation is on every one of the 9 labels read here — a property of the class rather than of any brand, so switching is not a way out of it. 5 more appear on some labels and not others, which is the part worth checking before you choose. Everything below is quoted from each product's own FDA prescribing information, with the date that label was published.
Last updated 2026-05-06 — the newest FDA label read here
What are the side effects of GLP-1 medications?
The FDA labels of the 9 weight-loss products tracked here agree on the core list: nausea, constipation, diarrhoea, vomiting. One mechanism explains most of it — these drugs slow how fast the stomach empties, which is how they produce the fullness that makes them work and also how they produce nausea, so the two arrive together rather than one being a malfunction. Listed on a label means it was seen often enough in a trial to be printed, not that it happens to everyone. Contrave, Foundayo, Mounjaro, Ozempic, Saxenda, Wegovy, Zepbound additionally carry a boxed warning, which is a different thing from a common reaction: a specific risk with a specific contraindication attached. The labels cannot say which drug is gentlest, and any page that ranks them is inventing the ranking: each reports its own trial under its own protocol and none was compared head to head, so a shorter list can mean a smaller trial. What is comparable is which reaction appears on which label, and the timing — the gut reactions cluster in the days after each dose increase, so how long they last depends on that drug's own titration schedule rather than on the class.
Which reaction is on which label
A tick means the reaction is named among the common adverse reactions on that product's own FDA label. An empty cell means it is not on that label — not that it never happens.
| Reaction | Contrave | Foundayo | Mounjaro | Ozempic | Phentermine | Qsymia | Saxenda | Wegovy | Zepbound |
|---|---|---|---|---|---|---|---|---|---|
| nausea | ✓ | ✓ | ✓ | ✓ | — | — | ✓ | ✓ | ✓ |
| fatigue | — | — | — | — | — | — | ✓ | ✓ | ✓ |
| constipation | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| diarrhoea | ✓ | ✓ | ✓ | ✓ | — | — | ✓ | ✓ | ✓ |
| vomiting | ✓ | ✓ | ✓ | ✓ | — | — | ✓ | ✓ | ✓ |
| hair loss | — | — | — | — | — | — | — | — | ✓ |
| injection site reactions | — | — | — | — | — | — | ✓ | — | ✓ |
hair loss appears on Zepbound's label alone — which is the kind of difference worth carrying into a conversation with a prescriber.
The boxed warnings
The FDA's most serious label warning. Not a common side effect — a specific risk with a contraindication attached.
Contrave
Bupropion raises the risk of suicidal thoughts and behaviour in children, adolescents and young adults. The label carries that as a boxed warning and tells prescribers to watch for mood changes, particularly in the first months and after any dose change.
Foundayo
In rodents this molecule caused thyroid C-cell tumours at human-relevant doses. Whether it does so in people is unknown, and the label carries that as a boxed warning. It is contraindicated outright with a personal or family history of medullary thyroid carcinoma or MEN 2.
Mounjaro
In rodents this molecule caused thyroid C-cell tumours at human-relevant doses. Whether it does so in people is unknown, and the label carries that as a boxed warning. It is contraindicated outright with a personal or family history of medullary thyroid carcinoma or MEN 2.
Ozempic
In rodents this molecule caused thyroid C-cell tumours at human-relevant doses. Whether it does so in people is unknown, and the label carries that as a boxed warning. It is contraindicated outright with a personal or family history of medullary thyroid carcinoma or MEN 2.
Saxenda
In rodents this molecule caused thyroid C-cell tumours at human-relevant doses. Whether it does so in people is unknown, and the label carries that as a boxed warning. It is contraindicated outright with a personal or family history of medullary thyroid carcinoma or MEN 2.
Wegovy
In rodents this molecule caused thyroid C-cell tumours at human-relevant doses. Whether it does so in people is unknown, and the label carries that as a boxed warning. It is contraindicated outright with a personal or family history of medullary thyroid carcinoma or MEN 2.
Zepbound
In rodents this molecule caused thyroid C-cell tumours at human-relevant doses. Whether it does so in people is unknown, and the label carries that as a boxed warning. It is contraindicated outright with a personal or family history of medullary thyroid carcinoma or MEN 2.
Every label, and every reaction on its own page
Contrave side effectslabel published 2025-11-17
nausea, constipation, headache, vomiting, dizziness, insomnia
Foundayo side effectslabel published 2026-08-13
nausea, diarrhoea, vomiting, constipation, indigestion, abdominal pain
Mounjaro side effectslabel published 2026-07-30
nausea, diarrhoea, reduced appetite, vomiting, constipation, indigestion
Ozempic side effectslabel published 2026-08-19
nausea, vomiting, diarrhoea, abdominal pain, constipation
Phentermine side effectslabel published 2026-08-19
raised heart rate, raised blood pressure, insomnia, dry mouth, constipation, restlessness
Qsymia side effectslabel published 2026-04-24
tingling in the hands and feet, dizziness, altered taste, insomnia, constipation, dry mouth
Saxenda side effectslabel published 2026-06-15
nausea, low blood sugar, diarrhoea, constipation, vomiting, headache
nausea →fatigue →constipation →diarrhoea →vomiting →injection site reactions →
Wegovy side effectslabel published 2026-06-30
nausea, diarrhoea, vomiting, constipation, abdominal pain, headache
Zepbound side effectslabel published 2026-05-06
nausea, diarrhoea, vomiting, constipation, abdominal pain, indigestion
nausea →fatigue →constipation →diarrhoea →vomiting →hair loss →injection site reactions →
GLP-1 side effects, answered
- What are the side effects of GLP-1 medications?
- On the FDA labels of the 9 products tracked here, the reactions that appear on nearly all of them are nausea, constipation, diarrhoea, vomiting. They come from the same mechanism: these drugs slow how fast the stomach empties, which is how they produce fullness and also how they produce nausea. Every one of them is listed because it was seen often enough in trials to be printed, not because it happens to everyone.
- Which GLP-1 has the fewest side effects?
- The labels do not answer that, and a page that ranks them is inventing the ranking. Each label reports its own trial under its own protocol, and the products were never compared head to head, so a shorter list can mean a smaller trial rather than a gentler drug. What is comparable is which reaction appears on which label, which is the table above.
- Which of these carry a boxed warning?
- Contrave, Foundayo, Mounjaro, Ozempic, Saxenda, Wegovy, Zepbound. A boxed warning is the FDA's most serious label warning and it is not the same as a common side effect — it is a specific risk with a specific contraindication attached, set out on each drug's own page here.
- How long do the side effects last?
- For the gut reactions, they cluster in the days after each dose increase and settle in between, so the pattern is a few rough days per step rather than one continuous stretch — which means the honest answer depends on that drug's titration schedule. A drug reaching its maintenance dose at week 5 stops stepping up long before one that takes 17. Each symptom page here sets the reaction against its own drug's published schedule.
Every cell is read from that product's own FDA prescribing information, linked on its page with the date the label was published. Information only, not medical advice — a symptom that worries you is a call to your prescriber, not a search result.