Two injectable medications, Mounjaro (tirzepatide) and Wegovy (semaglutide), have become prominent in managing obesity by suppressing appetite and aiding weight reduction. These drugs are typically administered weekly via pre-filled pens and are prescribed to adults with significant obesity-related health risks.
Both medications act by mimicking the hormone GLP-1, which influences feelings of fullness. Mounjaro additionally affects metabolism and energy regulation. Treatment usually begins at a lower dose, gradually increased to maintain effectiveness.
Clinical trials comparing the two found Mounjaro to offer greater average weight loss. Over 72 weeks, patients using Mounjaro achieved approximately 20% weight reduction, compared with 14% with Wegovy. These results are based on studies including obese individuals with an average weight of 113kg (nearly 18 stone).
Patients should use these medications alongside dietary and physical activity programmes, as recommended by the National Health Service (NHS). Eligibility for NHS prescriptions includes having a body mass index (BMI) over 35 with related health complications, or a BMI of 40 or more with specific coexisting conditions.
However, evidence indicates that weight regain is common after stopping these drugs. Research published in the British Medical Journal in 2026 showed that most patients regain lost weight rapidly, often four times faster than those who stop traditional dieting and exercise regimens. This presents a challenge for maintaining long-term results.
Further studies, including one funded by Eli Lilly involving a daily oral medication called orforglipron, suggest that follow-up treatment may help reduce weight regain. The drug is used in the US and may be introduced in the UK, although its long-term usage requirements remain under study.
Side effects common to GLP-1 receptor agonists include nausea, vomiting, bloating and gastrointestinal disturbances. These effects tend to subside over time but can lead some patients to discontinue treatment. Rarely, more serious adverse events such as gallbladder issues, kidney problems, or effects on the pancreas have been reported, though causality has not been conclusively established.
The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) advises against use of these medications during pregnancy, breastfeeding, or when trying to conceive due to insufficient safety data. Additionally, these drugs may interfere with the effectiveness of contraceptive pills, necessitating alternative methods.
Obesity, defined by excess body fat and typically measured by BMI, affects around one quarter of UK adults. The NHS uses BMI thresholds to guide treatment eligibility, adjusting for different ethnic backgrounds where appropriate. Despite some limitations, BMI remains a practical tool to identify individuals who may benefit from medical intervention for weight management.
In summary, Mounjaro and Wegovy provide effective options for weight loss in patients with obesity-related health risks but require careful consideration regarding long-term use and support to prevent rapid weight regain.