Methotrexate

Key facts

• For autoimmune conditions such as rheumatoid arthritis or psoriasis, methotrexate is taken as a single dose ONCE A WEEK, on the same day each week, never daily; accidental daily dosing has caused fatal overdoses.
• In cancer treatment, doctors use much higher doses on entirely different schedules under close specialist supervision. Never confuse the two regimens.
• Methotrexate can severely harm a developing baby and must not be used in pregnancy by either partner trying to conceive; reliable contraception is required during treatment and for months afterward.
• Regular blood tests checking bone marrow function, liver, and kidneys are mandatory throughout treatment.

What methotrexate treats

At low, once-weekly doses, methotrexate treats rheumatoid arthritis, psoriasis, psoriatic arthritis, and other autoimmune conditions. At much higher oncology doses, it treats certain cancers, including some leukemias and lymphomas, and it also ends an ectopic pregnancy. The dose and schedule for cancer treatment are entirely different from autoimmune use, and the two must never be mixed up.

How methotrexate works

Methotrexate blocks an enzyme cells need to use folate, a vitamin required to build new DNA. Fast-dividing cells, including overactive immune cells and cancer cells, are affected most. At low weekly doses this calms an overactive immune system; at high oncology doses it kills cancer cells outright. Because the drug also affects bone marrow and gut lining, it needs careful dosing and monitoring at every level.

Before you take it

• Confirm your exact dose and how often to take it with your pharmacist; if you're on a weekly autoimmune regimen, mark the day on a calendar.
• Do not take methotrexate if you are pregnant or breastfeeding; it causes birth defects and pregnancy loss. Reliable contraception is needed during treatment and after stopping, for both men and women.
• Tell your doctor about liver or kidney disease, low blood counts, or a history of lung problems.
• Folic acid is usually prescribed alongside weekly methotrexate to reduce side effects; take it exactly as directed.
• Avoid NSAIDs and antibiotics such as trimethoprim without checking first, since they can raise methotrexate to toxic levels.

Side effects

Common effects include nausea, mouth sores, fatigue, and hair thinning.

Stop and seek urgent medical care for any of these:

• Fever, sore throat, or unusual bruising or bleeding (possible dangerously low blood counts).
• Yellowing of the skin or eyes, dark urine, or severe abdominal pain (possible liver damage).
• Shortness of breath or a persistent dry cough (possible lung toxicity).
• Any sign that a weekly dose may have been taken daily by mistake; this is a medical emergency.

Safety essentials

• The single most important rule: weekly-dose methotrexate for autoimmune disease is taken once a week, never every day. Taking a daily dose meant to be weekly is a well-documented cause of fatal bone-marrow failure.
• Blood tests for full blood count, liver function, and kidney function are required before starting and at regular intervals throughout treatment.
• Methotrexate is teratogenic: it must be avoided in pregnancy and for a washout period before conception in both men and women, as your doctor advises.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.