Methotrexate: the pharmacology behind medical treatment for ectopic pregnancy.

Previously established as a chemotherapeutic agent for decades, methotrexate has been adapted for use as a medical therapy for unruptured ectopic pregnancies. Its mechanism of action, competitive inhibition of folate-dependent steps in nucleic acid synthesis, effectively kills the rapidly dividing ectopic trophoblast. However, the same action on normal cells is the basis for many of its adverse effects.