Wegovy Side Effects

Common and serious side effects of semaglutide, with evidence from clinical trials.

Common Side Effects (STEP 1 Trial)

The following side effects were reported in the STEP 1 clinical trial. Most are gastrointestinal and typically improve during the first 4–8 weeks of treatment.

Nausea~44%

The most common side effect. Usually mild and improves with time.

Diarrhoea~30%

Typically transient during dose escalation.

Vomiting~24%

More common during dose increases.

Constipation~24%

Increasing fibre and fluid intake can help.

Abdominal pain~20%

Usually mild. Severe pain needs urgent assessment.

Headache~14%

May relate to dietary changes and hydration.

Fatigue~11%

Can occur during dietary adjustment.

Dyspepsia~9%

Indigestion. Eating smaller meals helps.

Serious Side Effects

Acute pancreatitis

Severe persistent abdominal pain. Stop medication and seek immediate medical help.

Gallbladder disease

Gallstones and cholecystitis reported at higher rates. Watch for upper right abdominal pain.

Kidney injury

Can occur due to dehydration from severe GI symptoms. Stay hydrated.

Thyroid C-cell tumours

Observed in animal studies. Contraindicated in patients with personal/family history of MTC or MEN2.

Mental health changes

MHRA monitoring reports of mood changes. Report any concerns to your prescriber.

When to Seek Help

Call 999 for severe abdominal pain, allergic reactions, severe dehydration, or thoughts of self-harm. Contact your prescriber or NHS 111 for persistent vomiting, mood changes, or jaundice.

Managing Side Effects

  • Eat smaller, more frequent meals
  • Avoid greasy, fried, or heavy foods during dose escalation
  • Drink at least 2 litres of water daily
  • Eat slowly and stop when satisfied
  • Ginger tea may help with nausea
  • Ask your prescriber about slowing the titration if side effects are severe

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Important Medical Information

This website provides general information about weight loss medications for educational purposes only. It is not intended as medical advice, diagnosis, or treatment. Always consult your GP or qualified healthcare professional before starting any weight loss medication. Individual results may vary.

WhichWeightMeds.org is an independent comparison platform and is not affiliated with any pharmacy, manufacturer, or healthcare provider. We do not prescribe, sell, or dispense medications.

Sources & References

  1. 1
    Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1)New England Journal of Medicine, 2021 (Accessed February 2026)View source
  2. 2
    GLP-1 medicines for weight loss — what you need to knowMHRA (Accessed February 2026)View source