Treatment of obesity and overweight is based primarily on dietary measures and physical exercise.There are still no drugs with a favourable harm-benefit balance in this setting. Lorcaserin, a "selective" 5HT2C serotonin receptor agonist, has been refused marketing authorisation in the European Union despite approval in the United States. Clinical evaluation of lorcaserin is based on three placebo-controlled trials, each lasting one year, in a total of about 6000 patients. Two trials involved obese patients, and one obese patients with type 2 diabetes. The results of these trials are undermined by the large proportion (40% to 50%) of patients who were lost to follow-up before the end of the trial. None of the trials examined the impact of lorcaserin on the clinical complications of obesity. From an average initial weight of about 100 kg, patients taking lorcaserin lost only about 3 kg more than those in the placebo groups.The patients put on weight again after lorcaserin was discontinued. Adverse effects observed in clinical trials were mainly gastrointestinal (dry mouth, nausea) and neuropsychiatric (dizziness, fatigue, headache, euphoria). The incidence of cardiac valve disorders was higher with lorcaserin than with placebo. These trials were too short in duration to exclude a risk of cancer (breast cancer and astrocytoma) that was reported in experimental animals. This serotonin agonist is metabolised by the liver, creating a risk of multiple drug interactions. In practice, lorcaserin has not been shown to prevent complications of obesity or even lead to substantial weight loss.There is therefore no justification for exposing patients to the risk of adverse effects.