Educational guide
Weight loss medication side effects.
Every medicine can cause side effects. This guide sets out the common and serious ones reported with licensed weight loss medicines, and what to do if you’re worried.
Common side effects
With medicines that act on appetite hormones (such as GLP-1 receptor agonists), the most common side effects affect the digestive system, especially when starting or increasing the dose:
- feeling sick (nausea) or being sick
- diarrhoea or constipation
- stomach pain, bloating, wind or indigestion
- reduced appetite, headache, tiredness or dizziness
With medicines that reduce fat absorption, side effects include oily stools, urgency and wind, particularly after fatty meals.
Serious side effects: get urgent help
Call 999 or go to A&E, or contact NHS 111 urgently, if you have:
- severe stomach pain that doesn’t go away, sometimes spreading to your back — this can be a sign of pancreatitis
- pain in the upper right of your tummy, yellowing skin or eyes, or a high temperature — possible gallbladder problems
- signs of severe dehydration, such as being unable to keep fluids down
- signs of a serious allergic reaction, such as swelling of the lips, mouth or throat, or difficulty breathing
Pregnancy and contraception
Weight loss medicines that act on appetite hormones should not be used in pregnancy. The MHRA advises using effective contraception while taking them. Some may make oral contraceptive pills less effective, particularly when starting or increasing the dose — ask a pharmacist or prescriber for advice.
Surgery and sedation
Tell the healthcare team before any operation or procedure under anaesthetic or deep sedation, because these medicines slow stomach emptying.
Reporting side effects
You can report any suspected side effect to the MHRA using the Yellow Card scheme. This helps keep medicines safe for everyone.
Frequently asked questions
Do side effects go away?
Digestive side effects are often worst when starting or increasing a dose and may ease over time. Speak to your prescriber or pharmacist if they don’t, or if you’re worried.
How do I report a side effect?
Use the MHRA Yellow Card scheme online or through the Yellow Card app. You can report a side effect yourself; you don’t need a healthcare professional to do it.
Sources and further reading
- Semaglutide — NHS
- Tirzepatide — NHS
- Orlistat — NHS
- Yellow Card scheme: report a side effect — MHRA
- Medicines and Healthcare products Regulatory Agency — GOV.UK
This page is for general education only. It isn’t medical advice, it doesn’t recommend any treatment, and it doesn’t replace a conversation with a qualified healthcare professional about your own circumstances. If you need urgent medical help, call 999 or NHS 111.