People who start using anti-obesity medication one month after beginning behavioural therapy can lose double the amount of weight than those waiting the recommended six months.
A new study from the Perelman School of Medicine at the University of Pennsylvania has found that the addition of anti-obesity medications is beneficial for people who do not lose weight with behavioural methods alone.
Individuals living with obesity are at greater risk of developing type 2 diabetes, heart disease and certain types of cancer, prior research has reported.
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Currently, experts advise people needing to lose weight to try lifestyle interventions for at least six months as the first line of treatment.
Behavioural therapy programmes could include increased physical activity and a reduced-calorie diet.
Trained professionals often lead these interventions and help people set achievable health and weight goals.
Roughly half of people referred to a behavioural therapy programme do not achieve a clinically meaningful weight loss result, researchers have said.
First author Jena Shaw Tronieri said: “Surprisingly little is known about how to help patients who struggle to lose weight when they are already receiving frequent lifestyle counselling sessions.
“Some experts have suggested that adding an anti-obesity medication should be the next step, but no studies have tested whether this approach actually improves weight loss.”
During this trial, the team of researchers examined people who lost less than 2% of their initial body weight after a month of weekly behavioural sessions.
Each participant had to complete 24 more weeks of a behavioural intervention, but they were also randomly assigned to take either the anti-obesity medication phentermine hydrochloride (15.0 mg per day) or a placebo in this time.
The study has revealed that the participants who added phentermine lost 5.9% of their starting weight, compared to those in the placebo group only losing 2.8%.
Jena Shaw Tronieri said: “Our results strongly support the addition of anti-obesity medications for people who do not achieve meaningful weight loss with behavioural methods alone.
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“They also suggest that the medication can be introduced early in treatment, rather than waiting until a patient completes a full six-month programme.”
Jena Shaw Tronieri added: “Early intervention is crucial because patients who don’t see initial results are more likely to become discouraged and discontinue treatment altogether.”
Joint author Professor Thomas A Wadden said: “If the people who were early non-responders took one of the newer approved medications, like Semaglutide or tirzepatide, it’s likely they could easily double or triple their weight loss compared to phentermine. Additional research is needed to confirm this hypothesis.”
Read the study here: https://www.nature.com/articles/s41591-025-03556-3.
