Rates of antidepressant prescribing and obesity in the UK are rising simultaneously. As a clinical psychiatrist, I am familiar with the complaint from patients that antidepressant use is related to weight gain. Unfortunately, robust evidence on the strength of any association and its timing in relation to treatment inception is lacking. Nonetheless, any perception that weight gain is associated with antidepressant prescribing may lead to patients being less likely to continue with the antidepressant regime (and risk of subsequent relapse). More evidence based information could translate to greater adherence, a greater awareness of this potentially pervasive side effect, and, ultimately, to better outcomes.

Weight gain itself may sometimes be a symptom of serious psychiatric illness. Patients with serious psychiatric illnesses have a higher risk of weight gain through inactivity, anergia, and loss of motivation. While the association between antipsychotic drugs and weight gain is clearly described, the association between antidepressants and weight gain is less so. Patients who are prescribed antidepressants are therefore potentially exposed to an increased risk of weight gain by virtue of taking the antidepressant, having a mental illness, and being part of the UK population where obesity rates are rising annually. It is no wonder that so many of these patients put on weight that is resistant to weight management strategies.

Against this background of clinical need, and suspicion, I joined a study team investigating this association. The results of our analyses show that antidepressants are associated with an increased risk of weight gain—something I had long suspected. I was surprised, however, to find that this increased risk peaks about 2 to 3 years post-inception, when perhaps the focus on weight gain prevention strategies are no longer top of the list of clinical priorities. Further, this risk is elevated for up to 6 years. There is some evidence that particular antidepressants are more likely to be associated with weight gain than others.

Would patients be more likely to take their drugs if the association with weight gain were discussed with the prescriber in advance? We hope that these are the types of conversations that this research will stimulate.

This paper helps to facilitate a discussion on the strength of the association between antidepressant use in general and subsequent elevated risk of weight gain, the timing of this weight gain, and the potential association between individual antidepressants and increased risk of weight gain. The main analyses considered the risk of a 5% weight gain (3.5 kg; about half a stone for a 70 kg person), but we found evidence that this will sometimes be enough to move patients from the overweight to obese, or from obese to severely obese, categories. We hope that this article will enable patients and doctors to plan for the delayed risk of weight gain—which might be unexpected—and in doing so, enable better adherence to antidepressant prescribing and ultimately lead to better clinical outcomes. The potential impact of intervention at a patient level on the rise of obesity at a national level is obvious.

Rafael Gafoor is a researcher at King’s College London. He is a clinical psychiatrist with an interest in investigating the effectiveness of treatments for mental disorders.