How Mounjaro and Wegovy Function as Weight-Loss Medications
Mounjaro and Wegovy are two of the most popular weight-loss medications currently available in the UK, both through the NHS for certain patients and privately. These drugs are administered as weekly injections and have become widely used, with an estimated 1.6 million people in the UK on weight-loss drugs. Understanding how Mounjaro and Wegovy function helps clarify their role in managing obesity and related health conditions.
Mechanism of Action
Both Mounjaro and Wegovy work primarily by mimicking a hormone called glucagon-like peptide-1 (GLP-1). This hormone naturally helps regulate appetite by making people feel fuller, which reduces food intake. The medications are delivered via pre-filled injection pens that patients can self-administer into the upper arm, thigh, or stomach.
In addition to suppressing appetite, Mounjaro has an extra effect on metabolism and helps regulate energy balance, which may contribute to its effectiveness in weight loss. Patients typically start on a low dose that is gradually increased to a maintenance dose under medical supervision.
Effectiveness and Usage Guidelines
Clinical trials have shown that these medications can lead to significant weight loss. A head-to-head trial involving 750 obese participants found that Mounjaro resulted in an average weight reduction of 20% after 72 weeks, compared to 14% for Wegovy. This trial was funded by Eli Lilly, the manufacturer of Mounjaro.
The NHS recommends these injections for adults who are obese and have weight-related health risks, rather than for those seeking minor weight loss. For example, Wegovy is available on the NHS in England, Wales, and Scotland for adults with a body mass index (BMI) of at least 35 and at least one pre-existing weight-related health condition such as high blood pressure. It is also prescribed for some overweight patients with cardiovascular disease.
Both drugs are prescribed alongside lifestyle changes, including healthier eating and exercise programs. Patients usually begin to lose weight within a few weeks of starting treatment. The NHS limits the use of Wegovy to a maximum of two years.
Access and Availability
Mounjaro has been available through specialist NHS weight-loss clinics in England and Wales since early 2025, with limited availability in Scotland. Since mid-2025, some GPs in England have also been able to prescribe Mounjaro to patients meeting strict criteria, with incentives introduced in 2026 to increase access. It is estimated that around 3.4 million people in the UK could eventually qualify for Mounjaro, though full rollout may take over a decade according to the National Institute for Health and Care Excellence (NICE).
Both Mounjaro and Wegovy can also be purchased privately from supermarkets, chemists, and high-street clinics. However, the majority of users—around 90%—pay for these drugs privately. Prices for injection pens typically range from £100 to £300 depending on the dose. Since early 2025, online pharmacies have implemented stricter checks to prevent inappropriate sales, including verifying BMI and screening for eating disorders.
Side Effects and Safety Considerations
Common side effects of Mounjaro and Wegovy include nausea, vomiting, bloating, constipation, and diarrhoea. These effects are generally manageable and tend to diminish over time, though some patients discontinue use due to side effects. Genetics may influence how individuals respond to the medications.
More serious but rare side effects can include gallbladder and kidney problems, depression, and pancreatic issues, although no direct causation has been confirmed. The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has issued warnings advising women not to take these drugs during pregnancy due to unknown effects on unborn children. Additionally, the medications can interfere with the absorption of contraceptive pills, so alternative or additional contraception methods are recommended for women using these treatments.
Weight Regain After Stopping Treatment
Research published in the British Medical Journal indicates that many people regain most of the weight lost once they stop taking these medications, as normal food cravings return. Weight regain after stopping injections occurs more rapidly than after conventional dieting and exercise programs. This has led to calls for the NHS to provide ongoing support, including regular check-ups and advice, to help maintain weight loss.
A separate study published in Nature Medicine found that a daily oral medication called orforglipron, already available in the US and potentially launching soon in the UK, may help prevent weight regain after stopping injections. However, further research is needed to determine the duration of treatment required, which could potentially be lifelong.
Understanding Obesity and BMI Criteria
Obesity is defined as having excess body fat, and in the UK, about one in four adults is classified as obese. Body mass index (BMI) is the standard measure used to categorize weight status, although it has limitations since it measures weight relative to height and does not distinguish between fat and muscle mass.
The NHS uses BMI thresholds to determine eligibility for weight-loss medications, with lower BMI cutoffs recommended for people of Asian, Chinese, Middle Eastern, Black African, or African-Caribbean descent due to different health risk profiles.
