London, UK—Everything from the modern diet to chemicals in the environment has been blamed for long-term population weight gain. A new British study suggests a novel cause, however: widespread use of antidepressant medications.

The article in BMJ points out that patients prescribed any of the 12 most commonly used antidepressants are 21% more likely to experience an episode of weight gain than those not taking the drugs.

King’s College London researchers also determined that the increased relative risk of weight gain peaks after about 2 to 3 years of continued use, but for patients classified as being of normal weight, the risk of being reclassified to either the overweight or the obese category increased by 29% compared with those not taking antidepressants.

The risk of transitioning to the obese category was also 29% higher for already-overweight patients who were prescribed antidepressants versus those not on the drugs, the study found.

“The increasingly widespread use of antidepressants may be contributing to long-term population weight gain with associated health risks,” suggested coauthor Martin Gulliford, MA, FFPH.

Unlike previous studies—which have identified short-term weight gain following the initiation of antidepressant treatment—the new research illustrates how the risk of weight gain is elevated and sustained for multiple years after the start of treatment.

To reach those conclusions, the study team analyzed the 2004–2014 electronic health records of nearly 300,000 UK patients—136,762 men and 157,957 women.

The focus was on whether patients had been prescribed an antidepressant and included participants whose weight ranged from normal to super-obese, the latter of which was defined as a body mass index of 45 or greater.

Results indicate that, during 1,836,452 person-years of follow-up, the incidence of new episodes of 5% or more weight gain in participants not prescribed antidepressants was 8.1 per 100 person-years versus 11.2 per 100 person-years in those prescribed the drugs, for an adjusted rate ratio of 1.21.

“The risk of weight gain remained increased during at least six years of follow-up,” noted the researchers.

Meanwhile, in people who were initially of normal weight,  the adjusted rate ratio for transition to overweight or obesity was 1.29, and in participants who were initially overweight, the adjusted rate ratio for transition to obesity was also 1.29.

“Our results show that antidepressant treatment increases the risk of patients gaining weight over a period of years. From a clinical perspective, these observations reinforce the need for active, tailored and sustained body weight management to go hand in hand with the prescribing of antidepressant treatments,” noted lead author Rafael Gafoor, MD. “It’s important to stress that no patients should stop taking their medication and that if they have any concerns they should speak with their doctor or pharmacist.”