Methotrexate is an antimetabolite and immunosuppressant drug. It interferes with folate metabolism and DNA/RNA synthesis, which makes it useful in cancer chemotherapy but also at much lower doses for autoimmune diseases like rheumatoid arthritis, psoriasis and psoriatic arthritis. 

– Typical starting dose for rheumatoid arthritis/psoriasis: for example, ~7.5 mg once weekly (which may be 3 × 2.5 mg tablets) and then adjusted gradually up to around 15–20 mg once weekly depending on response and tolerability.

– In oncology use the dose is much higher and can be given daily or in cycles depending on regimen — but that is outside the low-dose tablet use discussed. 

– Your doctor will specify the exact day of the week to take the tablet. The same day each week is important.

– Monitoring (blood counts, liver/renal function, lung health) is required before and during therapy. 

Common Side Effects

Some of the more frequent or serious side-effects include:

• Nausea, vomiting, loss of appetite

• Mouth sores / stomatitis

• Hair thinning or hair loss

• Increased risk of infections (due to immunosuppression)

• Liver enzyme elevations / hepatotoxicity

• Bone marrow suppression (low white cells, red cells, platelets)

• Lung toxicity (pneumonitis)

• Because of teratogenicity — risk of fetal death or congenital anomalies if used in pregnancy. 

– Ensure you understand clearly that the tablet is taken once weekly, not daily (for non-oncology use). Errors in dosing schedule can be fatal. 

– Avoid in pregnancy (especially for rheumatoid/psoriasis indication) unless absolutely required; effective contraception required for both men and women of reproductive potential.

– Avoid live vaccines during therapy; avoid significant alcohol intake; monitor liver/renal/hematologic parameters.

– Avoid use if you have active severe infection, significant liver disease, severe blood disorders, ulcers of the mouth or GI tract, or serious lung disease (unless specialist supervises). 

– Folic acid supplementation: because methotrexate interferes with folate metabolism, folic acid/folinic acid is often prescribed to reduce side effects — but timing and regimen must be guided by your doctor. 

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