Mounjaro side effects

The common ones

In trials, roughly 1 in 3 to 1 in 2 people reported nausea at some point, and around 1 in 5 reported diarrhoea or constipation. These effects come directly from the mechanism: food leaves your stomach more slowly, so large or fatty meals sit heavily.

• Nausea, peaks in the 48 hours after an injection or dose increase. Cold, bland food and small portions help most.
• Constipation, slower gut transit plus less food volume. Fibre, fluids and daily walking are the first line. A pharmacist can suggest macrogol if needed.
• Diarrhoea, usually the mirror image of constipation and settles the same way. Hydration is the priority.
• Reflux and burping, a full-feeling stomach pushes acid upward. Smaller meals, not lying down after eating, and fewer fatty foods all help.
• Tiredness, often undereating in disguise. Check your protein and calories before blaming the medicine.
• Injection site reactions, mild redness or itching that fades in days. Rotating sites prevents most of it.

What eases them

• Serve half your usual portion and stop at the first sign of fullness.
• Eat slowly, put your fork down between mouthfuls.
• Cut fried food, cream and sweet food for the 3 days after each dose.
• Sip 2 litres of fluid across the day. Large gulps worsen nausea.
• Stay upright for an hour after eating.
• If a dose increase hits hard, ask about holding the current dose for 4 more weeks instead of stepping up. This is common and safe.

The rare but serious ones

Call 111 or see your GP promptly for pain under your right ribs with fever or yellowing skin or eyes (gallbladder problems), vomiting or diarrhoea you cannot keep fluids down through (dehydration risk), or symptoms of low blood sugar if you also take diabetes medicines, shaking, sweating, confusion.

People with diabetic retinopathy should have eyes checked before and during treatment, because rapid improvement in blood sugar can temporarily worsen it.

Hair shedding

Some people notice extra shedding a few months into rapid weight loss. This is telogen effluvium, a response to rapid loss itself, not a drug effect, and it regrows. Our hair loss guide explains the timeline and what helps.

Report every suspected reaction

Message the clinical team first, then report through the MHRA Yellow Card scheme. Reports from patients are how regulators spot rare problems early.