The pain of a migraine attack is believed to be due to greatly increased amplitude of pulsations in the cranial arteries, especially the meningeal branches of the external carotid artery. Ergotamine reduces extracranial blood flow, causes a decline in the amplitude of pulsation in the cranial arteries, and decreases hyperperfusion in the territory of the basilar artery. It does not reduce cerebral hemispheric blood flow.
Ergotamine produces constriction of both arteries and veins.
In doses used in the treatment of vascular headaches, ergotamine usually produces only small increases in blood pressure but it does increase peripheral resistance and decreased blood flow in various organs. The gravid uterus is particularly sensitive to these effects of ergotamine. Although specific teratogenic effects attributable to ergotamine have not been found, the fetus may suffer if ergotamine is given during pregnancy. Retarded fetal growth and an increased incidence of intrauterine death and resorption have been seen in animals. These are thought to result from ergotamine induced increases in uterine motility and vasoconstriction in the placental vascular bed.