Proxibarbal is a non-sedative barbiturate with a specific anti-serotonin and anti-histamine effect, due to enzyme induction of serotoninase and histaminase. Its lack of unpleasant side-effects permits its protracted use for the preventive treatment of migraine. Its chemical structure - 5-allyl-5-(beta-hydroxy-propyl)barbituric acid - allows enzyme induction within 1–3 months of treatment. Preventive treatment with 1–3 × 100 mg/day proxibarbal for 3 months eliminated migraine attacks in 25 out of 35 cases (71%). In a double-blind study of 30 female and 5 male patients, treatment with 3 × 100 mg/day produced a drop of from 5–30 attacks to 0–1 attack per month and normalization of high urinary serotonin, 5-HIAA or histamine levels. The typical side-effects of anti-serotonin drugs and antihistaminics, especially sedation or dizziness and hyperorexia, were not observed. The mechanism of this effect was proved by accelerated and increased destruction of serotonin and histamine in vitro by the serum of the patients after treatment.