Methotrexate

Key points

• Methotrexate: folic acid antagonist — inhibits dihydrofolate reductase → ↓ DNA synthesis.
• Uses: rheumatoid arthritis, psoriasis, some cancers; given ONCE WEEKLY (daily dosing errors are dangerous).
• Side effects: myelosuppression, liver fibrosis, pneumonitis, teratogenicity.
• Monitoring: FBC, U&E and LFTs before treatment and regularly after; prescribe folic acid (e.g. 5 mg weekly, different day).
• Interactions: NSAIDs and trimethoprim increase toxicity.

Decision points in this pathway

• Toxicity suspected? (↓counts, deranged LFTs, cough/dyspnoea)

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