GLP-1 Side Effects

What to expect from Wegovy and Mounjaro — common side effects, how to manage them, and when to seek medical help.

Key Points

  • Most side effects are gastrointestinal (nausea, diarrhoea, constipation) and typically improve over time
  • Gradual dose escalation (starting low and increasing slowly) is designed to minimise side effects
  • Serious side effects are rare but important to recognise — see the "When to seek help" section below

Common Side Effects

These affect more than 1 in 10 people taking GLP-1 treatments. They are usually mild to moderate and most common during the first weeks of treatment or when your dose increases.

Nausea

The most commonly reported side effect. Usually worst in the first 1-2 weeks at each new dose level.

Management tips:

  • Eat smaller, more frequent meals
  • Avoid fatty, greasy, or spicy foods
  • Eat slowly and stop when you feel full
  • Stay hydrated — sip water throughout the day
  • Try injecting at bedtime

Diarrhoea and Constipation

GLP-1 treatments slow gastric emptying, which can affect bowel habits in both directions.

Management tips:

  • Increase fibre intake gradually
  • Drink plenty of water (at least 2 litres daily)
  • Regular gentle exercise helps digestion
  • Over-the-counter remedies may help — ask your pharmacist

Vomiting

Less common than nausea but reported by some patients, particularly during dose escalation.

Management tips:

  • Follow the same dietary tips as for nausea
  • If vomiting persists, contact your prescriber
  • Watch for signs of dehydration (dark urine, dizziness)
  • Your prescriber may slow dose escalation

Other Common Effects

  • Headache — usually mild and temporary, particularly in the first week
  • Abdominal pain — typically mild cramping that improves with time
  • Fatigue — may occur as your body adjusts to lower calorie intake
  • Injection site reactions — redness, itching, or bruising at the injection site
  • Dizziness — particularly if you're not eating enough or are dehydrated

Serious Side Effects

Serious side effects are rare but important to recognise. If you experience any of these, contact your prescriber immediately or call NHS 111.

Pancreatitis

Severe, persistent abdominal pain (often radiating to the back) that doesn't go away. This is a medical emergency. Stop the medication and seek immediate medical attention.

Gallbladder Problems

Rapid weight loss can increase the risk of gallstones. Symptoms include sudden, intense pain in the upper right abdomen, pain between the shoulder blades, nausea or vomiting. Seek medical attention if you experience these.

Severe Allergic Reaction

Swelling of the face, lips, tongue, or throat, difficulty breathing or swallowing, severe rash. Call 999 immediately.

Kidney Problems

Usually caused by dehydration from vomiting or diarrhoea. Symptoms include decreased urination, swelling in legs or feet, feeling unusually tired. Stay hydrated and contact your prescriber if you can't keep fluids down.

Thyroid warning: Both semaglutide and tirzepatide carry a boxed warning about medullary thyroid carcinoma (MTC), based on findings in animal studies. They are contraindicated in people with a personal or family history of MTC or Multiple Endocrine Neoplasia syndrome type 2 (MEN2). Your prescriber will ask about this during your consultation.

When to Seek Help

Contact your prescriber

  • Side effects not improving after 4-6 weeks
  • Unable to eat enough to maintain nutrition
  • Persistent heartburn or acid reflux
  • Mood changes or low mood
  • Unexpected hair thinning

Call NHS 111

  • Persistent vomiting (can't keep fluids down)
  • Signs of dehydration
  • Severe constipation lasting more than a week
  • Upper abdominal pain lasting more than a few hours

Call 999 / go to A&E

  • Severe abdominal pain not going away
  • Difficulty breathing or swallowing
  • Swelling of face, lips, or throat
  • Signs of severe allergic reaction

Side Effects FAQ

How long do GLP-1 side effects last?

Most gastrointestinal side effects (nausea, diarrhoea, constipation) are worst during the first few weeks and during dose increases. They typically improve as your body adjusts. If side effects persist beyond 4-6 weeks at a stable dose, speak to your prescriber — they may adjust your dose or suggest management strategies.

Can I reduce nausea from Wegovy or Mounjaro?

Yes. Eating smaller meals, avoiding high-fat or greasy foods, eating slowly, and staying hydrated can all help. Some prescribers recommend anti-nausea medication during dose escalation. Taking your injection at bedtime rather than in the morning may also help, as the peak nausea may occur while you sleep.

Is hair loss a side effect of GLP-1 treatments?

Hair thinning (telogen effluvium) has been reported by some patients, though it is not listed as a common side effect in clinical trial data. It is more likely related to rapid weight loss itself rather than the medication directly. Hair typically regrows once weight stabilises. If you experience significant hair loss, discuss it with your prescriber.

What should I do if I experience severe side effects?

Stop taking the medication and contact your prescriber immediately if you experience: severe abdominal pain that doesn't go away (possible pancreatitis), signs of an allergic reaction (swelling of face/throat, difficulty breathing), persistent vomiting or diarrhoea leading to dehydration, or vision changes. In an emergency, call 999 or go to A&E.

Are side effects different between Wegovy and Mounjaro?

The side effect profiles are broadly similar — both primarily cause gastrointestinal symptoms. Some clinical data suggests tirzepatide (Mounjaro) may cause slightly more nausea at higher doses due to its dual mechanism, but individual experiences vary significantly. Your prescriber can help you choose based on your medical history and tolerance.

Sources

  • MHRA Summary of Product Characteristics — Wegovy (semaglutide 2.4mg)
  • MHRA Summary of Product Characteristics — Mounjaro (tirzepatide)
  • Wilding JPH et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity." NEJM 2021; 384:989-1002
  • Jastreboff AM et al. "Tirzepatide Once Weekly for the Treatment of Obesity." NEJM 2022; 387:205-216
  • NHS — Weight loss injections: patient information