GLP-1 side effects: what to expect in the first 8 weeks

What most people notice

Nausea is the most common effect, in trials, around 1 in 3 to 1 in 2 people reported it at some point. Constipation, diarrhoea, reflux and tiredness follow close behind. These come from the way the medicine works: food leaves your stomach more slowly, so large or fatty meals sit heavily.

Timing is predictable. Symptoms peak in the 48 hours after an injection or dose increase, then fade. By the end of each 4-week dose step, most people feel normal.

Five habits that ease symptoms

• Serve half your usual portion and stop at the first sign of fullness.
• Eat slowly, put your fork down between mouthfuls.
• Cut back on fried food, cream and sweet food for 3 days after each dose.
• Drink 2 litres of fluid a day in small sips, not large gulps.
• Stay upright for an hour after eating to reduce reflux.

Constipation, specifically

Slower gut transit plus smaller food volume makes constipation the most persistent effect for many people. Add fibre from vegetables, oats and pulses. Walk daily. And ask any pharmacist about a gentle osmotic laxative such as macrogol if you need one. Tell your prescriber if you go more than 3 days without a bowel movement.

When to hold your dose

If symptoms still disrupt daily life as your next increase approaches, tell your prescriber. Staying on the same dose for another 4 weeks is common, safe and often the fastest route to a comfortable higher dose later.

Warning signs

Call 999 or go to A&E for severe stomach pain spreading to your back, swelling of the face or throat, or trouble breathing. Call 111 for vomiting lasting more than 24 hours, pain under your right ribs, or yellow skin or eyes.

Report every suspected reaction

Message your clinical team, then report through the MHRA Yellow Card scheme. Patient reports are how regulators keep these medicines safe.