When given orally or intramuscularly in relatively low dosages methotrexate, a folic acid antagonist, can effectively suppress the signs and symptoms of psoriasis. But it neither cures the disease nor induces remissions in most cases. Physicians first used the drug only in severe incapacitating forms of psoriasis such as erythroderma, generalized pustular psoriasis, or other totally disabling psoriatic disease; now indications are expanding to include milder degrees of involvement where only cosmetic inconvenience exists.
Folic acid antagonists seem to exert their beneficial effects, both on benign and malignant diseases, by suppressing nucleic acid synthesis and preventing cell duplication. And these effects are exaggerated in rapidly dividing tissues such as those of the bone marrow, gastrointestinal epithelium, hair matrix, testis, and fetal tissues, as well as in psoriatic epidermis and in selected primitive and rapidly growing cancers. Methotrexate also has poorly delineated immunosuppressive and anti-inflammatory actions which are not understood. Recently,
Psoriasis, Methotrexate, and Cirrhosis. JAMA. 1970;212(2):314–315. doi:10.1001/jama.1970.03170150068015