Medicines and obesity: weight-loss treatment and wider effects

The NHS describes weight management as combining diet, activity and, for some people, medicines. Depending on body mass index and on other health conditions affected by weight, a person may be able to have a medicine to help them lose weight. These are not first-line on their own and are usually prescribed alongside diet and lifestyle advice.

After treatment stops, the NHS says people should be offered support to help them stay at a healthy weight. That recognises that the medicine is one part of a longer plan and that stopping it without a plan can be followed by weight regain.

The NHS groups weight loss medicines by how they work. One group reduces appetite and helps people feel fuller for longer, namely GLP-1 and GIP agonists such as semaglutide, tirzepatide and liraglutide. Another stops the body absorbing as much fat from food, which is orlistat.

These differ in side effects, monitoring and who they suit. A prescriber chooses between them and between the NHS and private routes, and the licence for each product sets out when it should be reviewed. Single-medicine details belong in each product's own information.

The MHRA reminds prescribers that these products are prescription-only and to be used under medical supervision. It says they are licensed for weight management in people with a body mass index above 30, or above 27 where there are weight-related conditions, and that the benefits and risks of using them for weight loss in people without obesity have not been studied.

It also warns that gastrointestinal effects such as nausea, vomiting and diarrhoea can cause severe dehydration leading to hospital admission and potentially kidney damage, advising people to stay well hydrated. It flags misuse for aesthetic weight loss and falsified products that may contain harmful substances.

The NHS tirzepatide page says additional barrier contraception is needed for four weeks after starting and after each dose increase, because tirzepatide may reduce the effectiveness of oral contraceptives. It states that tirzepatide is not used in pregnancy and that people should stop at least four weeks before trying to conceive.

The same page notes interactions with diabetes medicines including metformin, gliclazide and insulin, with warfarin and with anaesthetics, and asks people to tell professionals about all medicines, supplements and herbal remedies. Other products have their own cautions, so read the specific NHS page for any medicine you are prescribed.

Body size alters how some medicines are distributed in the body, and obesity is associated with conditions such as high blood pressure, diabetes and fatty liver disease that bring their own medicine considerations. Official pages for particular medicines state when weight matters, for example in dosing by weight in children, so prescribers check the product information for that medicine.

This page does not list dose adjustments, because the pages retrieved for this guide do not give general rules. Anyone who is significantly above or below average weight should say so when a new medicine is prescribed and ask whether the dose or monitoring plan has been considered.

The NHS notes that unregistered websites may sell counterfeit or unsuitable medicines and recommends checking that an online pharmacy is registered with the General Pharmaceutical Council. Prescription-only weight loss medicines should only be supplied after a prescriber has assessed you.

The MHRA's warning about falsified products matters for injectable pens in particular, where contents cannot be verified by the user. Report suspected side effects or suspect products to the MHRA through the Yellow Card scheme.