How does the effectiveness of Wegovy compare to phentermine?

Phentermine can produce modest, short-term weight loss, but it is not licensed in the UK for obesity treatment. In contrast, semaglutide 2.4 mg (Wegovy) has been shown in long-term studies to deliver sustained losses of around 15% and is specifically approved for weight management.

A different history and purpose

Phentermine and Wegovy are sometimes mentioned together because both have been used to support weight loss, but they come from very different backgrounds. Phentermine is a stimulant-like medicine, developed in the 1950s, that suppresses appetite by acting on the central nervous system. It is still used in parts of the United States but is not licensed in the UK for obesity treatment. Wegovy, by contrast, is a modern GLP-1 receptor agonist developed through extensive clinical trials and specifically approved by NICE for weight management.

Effectiveness of phentermine

Studies of phentermine have shown that it can produce short-term weight losses of 5–10% of body weight over three to six months. However, most trials have been small and limited in duration. Long-term data are lacking, and concerns about cardiovascular side effects have restricted its use in many countries. Because of this, phentermine is typically prescribed, where available, only for brief periods under close medical supervision.

Effectiveness of Wegovy

In contrast, semaglutide 2.4 mg (Wegovy) has been tested in large, long-term studies. In the STEP-1 trial, published in The New England Journal of Medicine, participants taking Wegovy lost on average about 15% of their body weight over 68 weeks, compared with about 2–3% in the placebo group. STEP-5 extended this evidence to two years, showing that weight loss could be sustained with ongoing treatment. These results demonstrate both stronger and longer-lasting effects than those typically seen with phentermine.

Safety and tolerability

The safety profiles of the two medicines also differ. Phentermine can raise heart rate and blood pressure, and its stimulant properties have led to concerns about dependence and misuse. These risks explain why it is not licensed in the UK. Wegovy’s main side effects are gastrointestinal, such as nausea or diarrhoea, especially during dose escalation. While unpleasant, these are usually manageable and lessen over time. Wegovy also has emerging evidence from the SELECT trial showing cardiovascular benefits in people with obesity and established heart disease — the opposite of the concerns linked with phentermine.

NHS and NICE perspective

Because phentermine is not licensed in the UK, it is not recommended by NICE or offered on the NHS. Wegovy, on the other hand, was approved by NICE in 2023 for use in adults with a BMI over 35 and weight-related conditions, or in some cases with a BMI over 30. It is now being rolled out across NHS specialist weight-management services. This difference reflects both the strength of evidence and the priority given to safety in national health guidance.

Why the comparison matters

Phentermine represents an older generation of weight-loss drugs: quick-acting but limited in safety and long-term benefit. Wegovy reflects a newer approach, supported by large-scale clinical evidence and approved for ongoing use within structured weight-management programmes. For people in the UK, the choice is not really between the two — phentermine is unavailable, while Wegovy is increasingly accessible through the NHS.

Why this detail matters

In effectiveness terms, Wegovy clearly outperforms phentermine for both the amount and the durability of weight loss. Just as importantly, it has an established safety record and NHS approval for long-term management. Phentermine may still have a role in certain countries for short-term use, but in the UK, Wegovy is the evidence-based option for sustainable weight control.