GLP-1 medications are among the most effective weight loss treatments ever developed — but they come with a well-documented set of side effects, particularly in the early weeks. Understanding what to expect, and how to manage it, is the difference between completing a programme and giving up prematurely.
The Most Common GLP-1 Side Effects
Clinical trials for both Mounjaro (tirzepatide) and Wegovy (semaglutide) report similar side effect profiles. The rates below are from SURMOUNT-1 (Mounjaro) and STEP-1 (Wegovy):
| Side Effect | Mounjaro (any dose) | Wegovy 2.4mg | |-------------|---------------------|--------------| | Nausea | 44% | 44% | | Diarrhoea | 25% | 30% | | Vomiting | 22% | 24% | | Constipation | 22% | 24% | | Injection site reaction | 14% | 15% |
The good news: most side effects are mild to moderate, peak during the first 4–8 weeks, and dramatically reduce as your body adapts.
Managing Nausea
Nausea is the most common reason people contact their GLP-1 provider in the first month. UK prescribers recommend:
- Eat small, bland meals — toast, rice, crackers, and plain chicken breast are easier to tolerate
- Avoid high-fat, greasy, or very spicy food, especially in the first 2–3 weeks
- Inject in the evening rather than the morning, so any peak nausea occurs while you sleep
- Stay hydrated — sip water slowly throughout the day
- Anti-nausea medication — if severe, your prescriber may suggest an over-the-counter antiemetic. Do not take prescription anti-nausea medication without medical guidance
Most patients find nausea reduces significantly within 4 weeks and is barely noticeable by week 8.
Managing Constipation
Constipation occurs because GLP-1s slow gastric emptying and gut motility. Practical tips:
- Increase fibre intake gradually (oats, vegetables, fruit)
- Stay well hydrated — aim for 2L of water daily
- Gentle movement, such as daily walking, helps gut motility
- A pharmacist-approved osmotic laxative (e.g. macrogol) may be recommended if constipation persists
Managing Diarrhoea
Some patients experience the opposite — loose stools, particularly in the first weeks. This usually resolves spontaneously. If it persists beyond 2 weeks or is causing dehydration, contact your prescriber.
Hair Loss (Telogen Effluvium)
Some patients report hair thinning or shedding after rapid weight loss — this is called telogen effluvium and is caused by the physical stress of rapid calorie restriction, not the medication itself.
It typically occurs 3–6 months after rapid weight loss and resolves spontaneously within 6–9 months. Ensuring adequate protein intake (at least 1.2g per kg of body weight daily) while on GLP-1 medication significantly reduces this risk.
Injection Site Reactions
Minor bruising, redness, or tenderness at the injection site is common. Rotate injection sites each week (abdomen → thigh → upper arm) and allow the pen to reach room temperature before injecting.
When to Contact Your Provider
Contact your UK prescriber or seek urgent medical attention if you experience:
- Severe, persistent abdominal pain radiating to the back (possible pancreatitis)
- Persistent vomiting preventing hydration
- Rapid heart rate at rest
- Vision changes
- Signs of allergic reaction (swelling, rash, difficulty breathing)
Does Dose Adjustment Help?
Yes — if side effects are severe, most UK prescribers will recommend holding at the current dose for an additional 4 weeks rather than increasing. This "extended titration" approach significantly reduces side effect burden without compromising long-term results.
Always communicate with your prescriber. Most reputable UK telehealth platforms (Voy, Juniper, Boots Online Doctor) have messaging systems for exactly this reason.