How do weight loss drugs like Mounjaro and Wegovy work?

Women using weight-loss jabs have been warned that they must use effective contraception because of fears the medicines could harm an unborn baby. Wegovy and Mounjaro jabs are currently only available on the NHS through specialist weight management clinics, but they can also be bought privately. The drugs will be offered to millions of the most obese people on the NHS over the next few years, but experts warn services are already struggling to deal with unprecedented demand. There are two drugs on the market - semaglutide, marketed under the brand name Wegovy, and tirzepatide, sold as Mounjaro. Semaglutide is also used in the type 2 diabetes treatment Ozempic. Both Wegovy and Mounjaro are given as weekly injections via pre-filled pens that can be self-administered into the upper arm, thigh or stomach. They work as an appetite suppressant by mimicking a hormone called glucagon-like peptide-1 (GLP-1). This is an intestinal hormone which is released after eating, and typically makes people feel fuller. Mounjaro also affects another hormone, glucose-dependent insulinotropic polypeptide (GIP), which affects metabolism and helps regulate energy balance. Patients generally start on a low dose which is gradually increased until they reach a higher maintenance dose. The UK's medicines regulator, the MHRA, says the lack of information about how the drugs could affect unborn children means the injections must not be taken: The drugs can also stop the contraceptive pill being absorbed properly, so the MHRA also recommends that some women taking the jabs use additional or alternative contraception. People taking these drugs typically start to lose weight within a few weeks. A first head-to-head trial of the two drugs suggested that Mounjaro is more effective than rival Wegovy. Both drugs led to substantial weight loss, but Mounjaro's 20% weight reduction, after 72 weeks of treatment, exceeded the 14% from Wegovy. The trial, which was paid for by Eli Lilly, the manufacturer of Mounjaro, involved 750 obese people, with an average weight of 113kg (nearly 18 stone). Previous clinical studies suggested that - when combined with changes to their diet, exercise and behavioural support - Wegovy users could lose more than 10% of their body weight after a year on the treatment. Other trials also suggested Mounjaro users might lose up to a fifth of their weight. Crucially, users of either drug can put weight back on once they stop treatment. Wegovy is available on the NHS in England, Wales and Scotland. It must be prescribed by a weight management specialist, and access to these is restricted. To qualify, adults must be obese (with a BMI of at least 35) and have at least one pre-existing weight-related health condition such as high blood pressure. People with cardiovascular disease who are classed as overweight, but not quite obese, can also be given the drug. Patients should follow a diet and physical activity programme, and the drug should be only prescribed for a maximum of two years, according to NHS guidelines. The body which assesses how all drugs are used, the National Institute for Health and Care Excellence (NICE), has recommended that Mounjaro should also be used alongside advice on diet and exercise. As with Wegovy, it will initially only be available to obese patients who have a weight-related health condition and are under the care of a specialist. Around 3.4 million people could eventually qualify for Mounjaro. To help manage demand,NICE has said the NHS can take more than a decade to roll out access to the drug. It is thought 300,000 could benefit in the first three years. Separately, the government had already announced plans to offer weight loss jabs to unemployed people in England living with obesity. Prime Minister Sir Keir Starmer said the movecould be "very important" for the economy and individuals' health. But weight-loss specialists warn that NHS obesity services are already struggling to deal with demand for these drugs, and that the injections should only be used as part of a wider treatment plan. Wegovy and Mounjaro can both be bought privately, including at some supermarkets, chemists and high-street clinics. Pens typically cost between £200 and £300, depending on the dose. The drugs should not be bought from non-regulated sources. A BBC investigation in November 2023found online sellers offering semaglutide as a medicine, without a prescription. It also found the drug being offered in beauty salons in Manchester and Liverpool. Under guidance introduced in February 2025,online pharmacies must employ stricter checksto stop people who are already a healthy weight or have a history of eating disorders buying weight-loss jabs. The BMI of patients who have completed online questionnaires or sent photos must be verified through in-person or video consultations, and checked with information from their GP or medical records. The most common side-effects are feeling sick, vomiting, bloating, constipation and diarrhoea. Some people report hair loss. For many the side-effects are manageable and tend to go away in time; others say they stopped taking the drugs because of them. In rare cases, serious side-effects can include gallbladder and kidney problems and depression. Experts warn the complications can be worse if the drugs are abused, for example, taken as a quick fix to lose a few pounds or bought from unregulated online sellers. Doctors say they have seen dangerous, life-threatening complications such as inflammation of the pancreas in such cases. That is why experts say they should only be used under medical supervision. Research suggestspeople do put most of the weight back on within a year of stopping the drugs, as their normal food cravings return. That is why experts recommend that patients improve their diet and increase the amount of exercise they take while on the treatment. Health Secretary Wes Streeting has warned about the danger of people developing a "dependency culture" on these drugs. In the past there have been reports of Wegovy shortages, although the manufacturer Novo Nordisk says the NHS has a protected supply. The term obese describes a person who has excess body fat. In the UK it's estimated that around one in every four adults is obese. The standard way of categorising people's weight is by calculating their body mass index, or BMI, which means dividing an adult's weight in kilograms by the square of their height in metres. For most adults, if your BMI is: Those with an Asian, Chinese, Middle Eastern, Black African or African-Caribbean family background need to use a lower BMI score to measure overweight and obesity: BMI has some limitations because it measures weight, not fat. Someone who is very muscular can have a high BMI without having excess fat. But for most people, it is a useful indication of whether they are a healthy weight.