This is an antihistamine drug that has a chemical structure similar to that of the tricyclic antidepressants, and also has similar anticholinergic effects. It is used to prevent migraine headaches in people who suffer frequent, disabling attack. The exact mechanism by which it works is not known, but it is thought to be due to its blocking action on chemicals (histamine and serotonin) that act on the blood vessels in the brain. Pizotifen has also been used to relieve the symptoms of carcinoid syndrome, a disorder in which excess production of serotonin causes attacks of flushing and diarrhoea. The main disadvantage of prolonged use of the drug is that it sometimes stimulates appetite and as a result often causes weight gain. It is usually prescribed only for people with whom simpler measures for the prevention of migraine (such as avoidance of stress and foods that trigger attacks) have failed.