Methotrexate

Methotrexate is an antifolate antimetabolite, a molecule closely resembling folic acid that competes with it inside cells ^[1][2]. It plays a dual role in medicine: at high doses it acts as a cytotoxic chemotherapy drug, and at much lower, once-weekly doses it serves as a disease-modifying antirheumatic drug and immunosuppressant ^[1][2]. Because it looks like folate, it binds to and disables the enzymes that cells need to use folate, thereby choking off the raw materials for making DNA ^[1][2].

The drug grew directly out of the earliest chemotherapy research. In the 1940s Yellapragada Subbarow synthesized folate antagonists, and in 1947 Sidney Farber and colleagues showed that the related compound aminopterin could induce remissions in childhood acute leukemia, the first demonstration that a drug could push such a cancer into retreat ^[1]. Methotrexate emerged around 1950 as a better-tolerated analogue, and it went on to cure choriocarcinoma and to treat a widening range of cancers and, later, autoimmune diseases ^[1].

In oncology methotrexate is used against cancers including leukemia, lymphoma, breast and lung cancer, osteosarcoma, and gestational trophoblastic disease ^[1]. In much smaller weekly doses it is a cornerstone treatment for rheumatoid arthritis, where controlled trials confirm meaningful improvement in symptoms and slowing of joint damage, and it is likewise used for psoriasis, psoriatic arthritis, and other inflammatory conditions ^[1][3]. It also serves to end an unruptured ectopic pregnancy and a molar pregnancy, and it is used in medical abortion ^[1]. To reduce its side effects, a folate supplement is usually co-prescribed with low-dose methotrexate, and in high-dose cancer regimens the antidote folinic acid, known as leucovorin, is given as a rescue to protect healthy tissue ^[1].

Methotrexate is a generic prescription medicine on the World Health Organization's Model List of Essential Medicines, available as tablets and as an injectable solution ^[1]. Its side effects follow from its action on dividing cells and include nausea, fatigue, mouth sores, suppression of blood-cell production, and a heightened risk of infection, while longer use can harm the liver and, less often, the lungs ^[1]. The drug is strongly teratogenic and must be avoided in pregnancy ^[1]. It also interacts dangerously with some medicines; for example, combining it with trimethoprim-sulfamethoxazole can cause serious toxicity even at the low doses used for arthritis, and it can blunt the immune response to some vaccines ^[1].

Methotrexate is prescription only and demands respect. The biggest safety rule is weekly dosing for autoimmune disease, because accidental daily dosing has caused fatal toxicity. Common effects include mouth sores, nausea, tiredness, and hair thinning, which daily folic acid helps prevent. Serious risks are liver damage, suppression of the bone marrow with low blood counts, lung inflammation, and kidney injury at high doses. It causes miscarriage and birth defects, so it is strictly avoided in pregnancy and reliable contraception is required. Interactions matter: NSAIDs and the antibiotic trimethoprim can raise methotrexate levels and toxicity, and live vaccines should be avoided. Regular blood tests track the liver, kidneys, and blood counts.

Methotrexate is cleared renally by tubular secretion, and anything competing for that route pushes exposure toward the toxic range. NSAIDs and salicylates are the classic offenders; they reduce renal clearance and displace methotrexate from albumin, and the result is myelosuppression, mucositis and acute kidney injury. Penicillins, probenecid and proton pump inhibitors do the same by different routes, and delayed elimination of high-dose methotrexate alongside PPIs is well described.

Trimethoprim-sulfamethoxazole is the most dangerous common pairing. Trimethoprim is itself a dihydrofolate reductase inhibitor, so the antifolate effect is additive while clearance falls at the same time; fatal pancytopenia has been reported even on low weekly rheumatology dosing.

Hepatotoxicity is additive with alcohol, leflunomide and azathioprine. Live vaccines are unsafe during treatment, because methotrexate suppresses the immune response that makes them safe.

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Methotrexate grew directly out of one of the founding moments of modern chemotherapy: in 1947 and 1948 the Boston pathologist Sidney Farber used the closely related antifolate aminopterin to induce the first temporary remissions of childhood acute leukemia, proving that a drug could halt cancer. Aminopterin proved difficult to work with, and chemists at Lederle Laboratories, in the group associated with Yellapragada Subbarow, synthesized the slightly modified analogue amethopterin, later renamed methotrexate, which offered a more manageable therapeutic window and became available for cancer treatment in the early 1950s.

Its potential in inflammatory disease was noticed early, when in 1951 Gubner reported that it improved both psoriasis and the arthritis that accompanied it, but decades passed before low weekly dosing was formally adopted; the United States Food and Drug Administration approved it for rheumatoid arthritis in 1988. Today it is a first-line disease-modifying drug for rheumatoid arthritis and a mainstay in psoriasis, several other autoimmune conditions, and the medical management of ectopic and molar pregnancy. It appears on the World Health Organization's list of essential medicines, a reflection of its enduring importance across oncology and rheumatology alike.

Methotrexate is one of the genuine landmark drugs of twentieth-century medicine, a molecule whose ancestor first showed the world that chemotherapy was possible and which then found a remarkable second life as the anchor of rheumatoid arthritis treatment. Rheumatologists regard low-dose weekly methotrexate as the gold-standard first-line agent, the drug against which newer and far more expensive biologics are benchmarked, and it retains a devoted following for its combination of durable efficacy and low cost.

Its versatility is extraordinary, spanning leukemia, lymphoma, psoriasis, and ectopic pregnancy from a single well-understood mechanism, and the availability of folinic acid "rescue" even allows very high doses to be given safely in cancer care. Honesty requires acknowledging that it demands respect: it needs regular blood monitoring, folate supplementation, and strict avoidance in pregnancy, and a widely cited cardiovascular prevention trial found it did not reduce heart events in patients without inflammatory disease. Within its established indications, however, it remains a trusted, essential, and heavily validated therapy.