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Long term use of antidepressants is associated with a sustained increase in risk of weight gain over at least 5 years, according to a study by UK researchers published in the BMJ. The findings show that patients prescribed any of the 12 most commonly used antidepressants were more likely to experience weight gain than those not taking the drugs. The researchers analysed body weight and body mass measurement data from the UK Clinical Practice Research Datalink (CPRD) for over 300 000 adults with an average age of 51 years, whose body mass index (BMI) had been recorded three or more times during GP consultations from 2004 to 2014. Participants were grouped according to their BMI (from healthy weight to severely obese) and whether or not they had been prescribed an antidepressant in a given year. Participants were then monitored for a total of 10 years. The researchers found that the absolute risk of gaining at least 5% weight without antidepressant use was 8.1 per 100 person years; whereas the risk with antidepressant use was 9.8 per 100 person years. This means that for every 59 people taking antidepressants, one extra person would gain at least 5% weight over the study period. The risk was greatest during the second and third years of treatment. During the second year of treatment, the risk of gaining at least 5% weight was 46% higher than in the general population, but no association was found during the first 12 months of treatment. This is an observational study, so no firm conclusions can be drawn about cause and effect. They also found that people who initially had healthy weight had a higher risk of moving to either the overweight or obese groups, and people who were initially overweight had a higher risk of moving to the obese group if they were taking antidepressants.