Telotristat ethyl (Xermelo^®), a first-in-class peripheral tryptophan hydroxylase (TPH) inhibitor, is approved to treat carcinoid syndrome diarrhoea in combination with somatostatin analogue (SSA) therapy in adults inadequately controlled by SSA therapy alone. Some neuroendocrine tumours secrete serotonin (5-HT) into the blood, resulting in frequent bowel movements (BMs) and other symptoms. Telotristat ethyl inhibits TPH, thereby reducing the production of 5-HT and improving carcinoid syndrome diarrhoea. In the 12-week placebo-controlled phase of randomized trials in patients with carcinoid syndrome diarrhoea (most of whom were receiving SSA therapy), the addition of oral telotristat ethyl 250 three times daily provided significant reductions in the frequency of BMs and levels of urinary 5-hydroxyindolacetic acid (u5-HIAA; a metabolite of 5-HT) relative to placebo. Telotristat ethyl 250 mg three times daily was well tolerated, with the proportion of patients reporting at least one treatment-emergent adverse event being similar to that with placebo. With regard to adverse events of special interest, relative to placebo, telotristat ethyl had a comparable incidence of depression-related symptoms, a somewhat higher incidence of gastrointestinal (GI) disorders and a higher incidence of elevated hepatic enzyme levels.