A trial which tested the effectiveness of an anti-obesity drug found that participants regained much of the lost weight within a year of stopping treatment, suggesting people may need to be on the medication long term.

The research also showed that tirzepatide, which works by signalling to the brain that a person is full up, helps to maintain improvements in metabolic and cardiovascular health in those who continue taking the medication.

The team behind the research say that because obesity is a chronic condition like diabetes or high blood pressure, “it must be treated chronically”.

The randomised trial, by an American team from Weill Cornell Medicine and NewYork-Presbyterian, found that while the drug can make a significant difference when it comes to helping people lose weight, it is not a quick fix.

Lead study author Dr Louis Aronne, the Sanford I. Weill Professor of Metabolic Research and director of the Comprehensive Weight Control Center at Weill Cornell Medicine, said: “People feel much better when they lose this kind of weight, so they are extremely enthusiastic about these treatments.

“But they also should realise this may require them to stay on the drug long term.”

The research team wanted to evaluate whether weight loss continues once treatment with tirzepatide comes to an end.

Their trial took place across 70 sites in Argentina, Brazil, Taiwan and the United States between March 2021 and May 2023.

For 36 weeks, the group received tirzepatide, which saw them lose around 20% of their body weight alongside improvements in blood pressure, blood sugar metrics and lipid levels.

During the next stage of the trial, 670 participants either continued with the treatment for another year or were given a placebo instead.

The group which remained on tirzepatide lost a further 5.5% of body weight, while the placebo group regained 14% of their weight.

The improvements in cardiometabolic risk factors were reversed among the placebo group, despite the fact that they were still 10% lighter than their starting weight.

Dr Aronne said: “Those who went on the placebo regained about half the weight they had lost. Whereas those who continued on the drug lost another 5%, so their overall weight loss was about 25%.

“If you stop the medication, you regain the weight. There’s no question that will happen. Obesity is a chronic condition, like diabetes or high blood pressure. So, it must be treated chronically.”

He added: “Obesity is a leading driver of many diseases that we spend our time treating in medicine; illnesses like hypertension, heart disease, diabetes and fatty liver disease are either caused by or worsened by obesity.

“The fact that we now have drugs that are proving to be effective is exciting and rewarding.”

Tirzepatide, with the trade name Zepbound, was recently approved by the US Food and Drug Administration for weight loss in people with a body mass index (BMI) of 30 or higher.

It can also be prescribed for people with a BMI of 27 or greater with health conditions including high cholesterol or hypertension.

Further research is needed to evaluate the long-term benefits and risks the drug poses, particularly if they are to be used for the duration of people’s lives.

Read the study in full in the journal JAMA.

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