Starting a GLP-1 medication like Wegovy (semaglutide), Ozempic (semaglutide) or Mounjaro (tirzepatide) almost always comes with some side effects — especially in the beginning. Knowing what to expect, when symptoms typically peak, and how to ease them can make a big difference in staying on track with your treatment. This article gives you a clear, evidence-based overview.
The Most Common Side Effects
GLP-1 medications work by slowing stomach emptying and signalling fullness to the brain. As a result, the digestive system bears the brunt of most side effects. According to the FDA's official prescribing label for Wegovy, 73% of patients reported gastrointestinal adverse reactions during treatment, compared to 47% on placebo.
Most common side effects and their rates in Wegovy trials:
| Side Effect | Wegovy | Placebo |
|---|---|---|
| Nausea | 44% | 16% |
| Diarrhoea | 30% | 16% |
| Vomiting | 25% | 6% |
| Constipation | 24% | 11% |
For Mounjaro, the rates are somewhat lower but the pattern is the same: nausea and diarrhoea occur in more than 1 in 10 users, with vomiting and constipation affecting up to 1 in 10.
Other reactions some people notice: heartburn, bloating, belching, hiccups, fatigue, and changes in taste.
When Do Side Effects Occur?
The good news is that side effects don't have to last the whole treatment. They are typically strongest during dose escalation — the first several months when your dose is gradually increased.
Wegovy dose escalation schedule (16 weeks to maintenance):
| Week | Dose |
|---|---|
| Weeks 1–4 | 0.25 mg |
| Weeks 5–8 | 0.5 mg |
| Weeks 9–12 | 1.0 mg |
| Weeks 13–16 | 1.7 mg |
| Week 17 onwards | 2.4 mg (maintenance) |
Mounjaro's escalation takes up to 20 weeks, starting at 2.5 mg and increasing by 2.5 mg every 4 weeks up to a maximum of 15 mg. Nausea and other digestive symptoms typically peak in the days after each dose increase, then ease off as the body adjusts.
Practical Tips to Manage Nausea and Digestive Issues
You can significantly reduce discomfort with some simple adjustments. These tips are backed by NHS guidance and Mayo Clinic recommendations:
- Eat smaller, more frequent meals. Large meals are much harder to tolerate on GLP-1 medications.
- Eat slowly. Put your fork down between bites. Stop when you feel comfortably full — not stuffed.
- Avoid fatty and fried foods. High-fat meals are particularly difficult to digest on these medications. Choose baked, steamed or boiled options instead.
- Eat bland foods when nauseated. Crackers, toast, plain rice or boiled potatoes are gentle on the stomach.
- Stay upright after meals. Avoid lying down for at least 1–2 hours after eating.
- Sip water throughout the day — not during meals. Drink between meals rather than with food. Ginger tea can also help with nausea.
- Inject before bedtime. Many people find that sleeping through the worst of the nausea makes evenings much more comfortable.
- Let the pen warm to room temperature. A cold injection can be more uncomfortable than one taken at room temperature.
What If Side Effects Feel Unbearable?
Talk to your doctor — you don't have to push through at all costs. There are official options for slowing the escalation:
- You can delay a dose increase by up to 4 weeks if side effects are too disruptive.
- Wegovy: If the 2.4 mg maintenance dose is not tolerated, you can temporarily drop back to 1.7 mg for up to 4 weeks before trying again.
- Mounjaro: Your prescriber can pause or reduce the dose during the escalation phase.
According to FDA data, 4.3% of Wegovy users stopped treatment due to gastrointestinal side effects. That is not a failure — it is a clinical decision, and dose adjustments often allow treatment to continue successfully.
Side Effects You Should Never Ignore
Most side effects are mild and temporary. However, some symptoms require urgent attention:
- Severe abdominal pain that radiates to the back — possible pancreatitis. Seek medical help immediately.
- A lump in the neck, difficulty swallowing, or persistent hoarseness — possible thyroid issue. Contact your doctor.
- Sudden vision loss or rapidly worsening eyesight — in 2025, the EMA classified a rare eye condition (NAION) as a possible side effect of semaglutide. Seek urgent eye care.
- Severe vomiting with signs of dehydration (dark urine, dizziness) — contact your doctor or NHS 111.
- Severe upper-right abdominal pain with fever — may indicate gallbladder problems.
Always tell your anaesthetist or surgeon that you are on GLP-1 medication before any procedure requiring general anaesthesia or deep sedation. These drugs slow stomach emptying and can increase the risk of aspiration.
Hair Loss and Fatigue — What Is Causing Them?
Some people experience hair thinning in the first months of treatment. This is usually due to rapid weight loss (a condition called telogen effluvium) rather than the medication itself, and it is typically temporary. Fatigue is also reported — making sure you eat enough protein and stay well hydrated can help.
The Bottom Line
Side effects from GLP-1 medications are real and can be frustrating — but for most people they are manageable and improve over time. The key insight is that they peak during dose escalation and typically ease once your body has adjusted to a stable dose. Use the practical tips above, and speak openly with your prescriber if things feel too difficult.
Sources
- FDA — Wegovy (semaglutide) Prescribing Label 2024
- FDA — Mounjaro (tirzepatide) Prescribing Label 2025
- EMA — NAION as very rare side effect of semaglutide (2025)
- Mayo Clinic — Semaglutide (subcutaneous route): Side effects & dosage
- Mayo Clinic — Tirzepatide (subcutaneous route): Side effects & dosage
- NHS Guy's & St Thomas' — GLP-1 agonists: patient information