The typical pattern (so you know you're normal)
Side effects on Mounjaro, Wegovy, Ozempic and the Wegovy pill follow a recognisable rhythm: they appear in the first few weeks, spike in the 24 to 72 hours after each dose, flare again at dose increases, and fade for most people within a month or two at a stable dose. In the big trials, GI side effects were common (the Wegovy pill's OASIS 4 saw them in 74% of participants) but mostly mild-to-moderate, and only 5 to 7% of people stopped treatment because of them.
The common ones, and what genuinely helps
Nausea (the most common)
- Smaller, plainer meals; cold or room-temperature food smells less and sits better.
- Strictly avoid greasy, fried and very rich food. The top trigger.
- Ginger (tea, biscuits, capsules) has real evidence behind it.
- Eat slowly; stop at satisfied. "Just a bit more" is the nausea threshold.
- Plan lighter days for the 1 to 2 days after your injection.
Constipation
- Fibre increased gradually (oats, beans, kiwis, vegetables) plus consistent fluids. See our hydration guides for Mounjaro and Wegovy.
- Daily movement, even a 20-minute walk, measurably helps gut transit.
- Persistent? Ask your pharmacist about gentle osmotic options. This is routine territory for them.
Diarrhoea
- Temporarily ease off fibre and rich food; small bland meals.
- Replace fluid and salts. Oral rehydration sachets if it's more than a day.
Reflux and indigestion
- Smaller evening meals, nothing rich within three hours of bed, head of bed raised if it's a pattern.
Fatigue, dizziness, hair shedding
- Fatigue and dizziness usually track undereating and under-drinking. Check protein (calculator) and fluids before blaming the drug.
- Temporary hair shedding (telogen effluvium) follows any rapid weight loss, typically appearing 2 to 4 months in and resolving as weight stabilises. Adequate protein and iron are the mitigation.
The red flags. Act on these
Especially pain radiating to your back with vomiting. The classic acute pancreatitis picture. The MHRA strengthened pancreatitis warnings for all GLP-1 and dual GLP-1/GIP medicines in 2026, including rare necrotising and fatal cases. Stop the medicine and seek urgent medical care (999/A&E for severe symptoms).
The MHRA flagged a rare risk of NAION (non-arteritic anterior ischaemic optic neuropathy. Sudden, painless vision loss) with semaglutide in February 2026. Sudden loss of vision in one or both eyes = emergency assessment.
- Right-upper-abdominal pain, fever or jaundice (gallbladder. GLP-1s increase gallstone risk, especially with fast loss)
- Signs of severe dehydration. Barely passing urine, faintness, confusion
- Allergic reactions. Swelling of face/throat, breathing difficulty (999)
- Persistent vomiting preventing fluid intake for 24 hours
- Low mood or thoughts of self-harm. The MHRA continues to monitor psychiatric safety; report changes to your prescriber
The surgery rule everyone forgets
Because GLP-1s slow stomach emptying, food can remain in the stomach even after standard fasting. A pulmonary aspiration risk during general anaesthesia. The MHRA issued a specific warning (January 2025). Always tell your anaesthetist and surgical team you take a GLP-1, well before any procedure. They may adjust fasting times or pause the medicine.
Report side effects. It genuinely matters
The MHRA Yellow Card scheme is how the safety warnings on this very page came to exist. Pancreatitis, NAION and aspiration warnings all emerged from accumulated reports. Ten minutes of your time protects the next person. Anyone can report: patients, carers, clinicians.
Frequently asked questions
How long do GLP-1 side effects last? +
For most people, the worst is the first 4 to 8 weeks and the days following each dose increase. At a stable dose, most GI effects settle. If side effects remain disruptive after two months at a stable dose, discuss dose adjustment with your prescriber rather than pushing through.
What is the most dangerous side effect of Mounjaro or Wegovy? +
Acute pancreatitis is the serious risk the MHRA has most emphasised. Watch for severe, persistent abdominal pain radiating to the back, especially with vomiting. For semaglutide specifically, sudden vision loss (NAION) is a rare emergency. Both need urgent medical care.
Does hair loss from GLP-1s grow back? +
Yes. The shedding seen with rapid weight loss (telogen effluvium) is temporary and typically regrows as weight stabilises and nutrition normalises. Adequate protein, iron and avoiding crash-level intake reduce the risk.
Do I need to stop my GLP-1 before surgery? +
Tell your anaesthetist you take a GLP-1 as early as possible. MHRA guidance warns of pulmonary aspiration risk because of delayed stomach emptying. Your team will decide whether to adjust fasting or pause the medicine; do not make that call alone.
How do I report a side effect in the UK? +
Via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk). It takes about 10 minutes, anyone can report, and these reports directly drive safety warnings like the pancreatitis and NAION updates.
References & official sources
Show all 6 cited sources, with study authors +
- MHRA Drug Safety Update. GLP-1 and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis: www.gov.uk/drug-safety-update
- MHRA Drug Safety Update. Semaglutide (Wegovy, Ozempic, Rybelsus): risk of NAION: www.gov.uk/drug-safety-update
- MHRA Drug Safety Update. GLP-1 receptor agonists: potential risk of pulmonary aspiration during general anaesthesia: www.gov.uk/drug-safety-update
- electronic medicines compendium. Mounjaro patient leaflet: www.medicines.org.uk/emc/search?q=mounjaro
- electronic medicines compendium. Wegovy patient leaflet: www.medicines.org.uk/emc/search?q=wegovy
- MHRA. Yellow Card reporting scheme: yellowcard.mhra.gov.uk/