Medicines can help with weight loss, but they work best alongside changes to eating and activity, not instead of them. Here’s how the main options compare. Only one is available without a prescription.
Over the counter: orlistat 60 mg (Alli)
- How it works: it blocks about a quarter of the fat you eat from being absorbed.
- Who it’s for: adults aged 18 and over with a BMI of 25 or more, together with a reduced-calorie, low-fat diet.
- How to take it: one capsule with each main meal that contains fat, up to three times a day.
- Side effects: oily spotting, wind with an oily discharge, and urgent bowel movements, especially after high-fat meals. Keeping to about 15 g of fat per meal reduces them.
- Important: take a multivitamin containing vitamins A, D, E and K at bedtime. Don’t use orlistat if you take ciclosporin, and ask a pharmacist if you take levothyroxine, warfarin or epilepsy medicine.
- Results: modest, typically a few extra pounds over a year compared with diet alone.
Prescription medicines
Doctors may prescribe a weight-loss medicine to adults with a BMI of 30 or more, or 27 or more with a weight-related condition such as type 2 diabetes, high blood pressure or sleep apnoea.
- Semaglutide (Wegovy) and tirzepatide (Zepbound): weekly injections that reduce appetite and slow stomach emptying. In trials, average weight loss was about 15% with semaglutide and around 20% with tirzepatide at the highest dose. The same medicines are sold for diabetes as Ozempic and Mounjaro. See Ozempic vs Mounjaro.
- Liraglutide (Saxenda): a daily injection from the same family, with smaller average weight loss.
- Phentermine-topiramate (Qsymia) and naltrexone-bupropion (Contrave): tablets that reduce appetite.
Common side effects of the injections include nausea, vomiting, diarrhoea and constipation, mostly when starting or increasing the dose. Rarer but serious risks include pancreatitis and gallbladder problems. They aren’t used in people with a personal or family history of medullary thyroid cancer or MEN2 syndrome, or in pregnancy.
Weight often returns after stopping, so these medicines are usually long-term treatments.
Avoid fake and unapproved products
The FDA has warned about counterfeit semaglutide and about unapproved “weight loss” products sold online, some containing the wrong dose or hidden drugs. Prescription weight-loss medicines should come from a licensed pharmacy with a valid prescription from a doctor who has assessed you. Be wary of any website that offers them without one.
What about supplements?
No supplement has good evidence for meaningful weight loss. Products sold for weight loss have been found to contain hidden prescription drugs, including some withdrawn for heart risks.
Start with the basics in how to lose weight safely.
More in our Weight Management guide: all our articles on this topic in one place.
This article is for general information only and isn’t a substitute for advice from a doctor or pharmacist. Always read the label before using any medicine.
