Methotrexate: what it treats and how to use it safely

One drug can calm severe autoimmune disease at low weekly doses and fight cancer at much higher doses. That drug is methotrexate. If you've been prescribed it, you probably have a lot of questions. This page explains, in plain words, what methotrexate does, typical side effects, how doctors monitor you, and the safety points that really matter.

What methotrexate is and when doctors use it

Methotrexate is a medicine that slows down cell growth and the immune system. At low doses it treats conditions like rheumatoid arthritis, psoriasis, and some inflammatory diseases. At higher doses it's used as chemotherapy for certain cancers. It comes as a tablet and as an injection (given into a muscle or vein).

Because the effect depends on dose, your doctor will tell you exactly how often and how much to take. For long-term immune conditions, a common pattern is a single weekly dose. For cancer, dosing schedules are very different and managed in hospital.

Side effects, monitoring, and safety tips

The most common side effects are nausea, mouth sores, hair thinning, and mild liver changes. More serious risks include low blood counts, liver damage, and lung inflammation. That's why doctors order regular blood tests: a full blood count and liver function tests first, then at intervals while you take the drug.

Take folic acid if your doctor prescribes it — it reduces some side effects like mouth sores and fatigue. Don’t double up if you miss a weekly dose; call your clinic. Avoid alcohol or keep it very limited, since alcohol raises the risk of liver harm.

Methotrexate can interact with other medicines. Common examples: some antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and certain diuretics can change how methotrexate behaves in the body. Tell your doctor or pharmacist about every medicine and supplement you take. Also avoid live vaccines while on methotrexate unless your doctor says otherwise.

Important safety rule: methotrexate can cause severe birth defects. If you are pregnant or planning to become pregnant, don’t use methotrexate. Both people assigned female and male at birth should discuss contraception with their doctor before treatment and for a time after stopping the drug.

Watch for warning signs and call your healthcare team if you get fever, sore throat, new shortness of breath, yellowing of the skin or eyes, severe vomiting, or unusually heavy bruising or bleeding. These can be signs of serious complications that need prompt review.

Finally, always follow the exact dosing schedule your clinician gives you, keep scheduled lab checks, and ask questions if a pharmacy or online seller offers different dosing or substitutes. Methotrexate works well for many people but needs respect and routine monitoring to stay safe.