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Disease-Modifying Antirheumatic / Immunosuppressant Pregnancy: Contraindicated — major teratogen; methotrexate PPP (Pregnancy Prevention Programme)

Methotrexate (Dermatology — Psoriasis)

Brand names: Methofar, Methofar XL

Adult dose

Dose: Psoriasis: 7.5–15 mg ONCE WEEKLY. Increase by 2.5–5 mg every 2–4 weeks. Target: 10–25 mg once weekly.
Route: Oral or SC injection
Frequency: ONCE WEEKLY ONLY — NEVER DAILY (fatal error)
Max: 25 mg once weekly
ALWAYS ONCE WEEKLY. Prescribe with folic acid 5 mg once weekly (different day to MTX). Monitor FBC, LFTs, and renal function.

Paediatric dose

Dose: 0.1 mg/week/kg
Route: Oral or SC
Frequency: ONCE WEEKLY
Max: 25 mg/week
Concentration: 2.5 mg tablet; 2.5–50 mg/mL SC solution mg/week/ml
Paediatric psoriasis/JIA: 0.1–0.4 mg/kg/week. ONCE WEEKLY. Folic acid co-prescribed.

Dose adjustments

Renal

Avoid if eGFR <30 — severe accumulation and toxicity

Hepatic

Avoid in hepatic impairment (hepatotoxic)

Paediatric weight-based calculator

Paediatric psoriasis/JIA: 0.1–0.4 mg/kg/week. ONCE WEEKLY. Folic acid co-prescribed.

Clinical pearls

  • ONCE WEEKLY — never daily. Patients have died from daily dosing errors. Yellow prescription alert card.
  • Folic acid 5 mg once weekly (not on same day as MTX) reduces mucosal and haematological toxicity
  • Contraception: men and women must use contraception during treatment and for 6 months after stopping
  • LFT monitoring: fibrosis risk — may require liver biopsy after cumulative dose >1.5 g if enzymes persistently elevated

Contraindications

  • Pregnancy
  • Breastfeeding
  • Significant renal impairment
  • Hepatic disease
  • Active infection

Side effects

  • Hepatotoxicity (cumulative fibrosis)
  • Myelosuppression
  • Mucositis (reduce with folic acid)
  • Pulmonary toxicity (rare, acute)
  • Teratogenicity
  • GI upset

Interactions

  • NSAIDs — reduce MTX clearance
  • Trimethoprim — additive anti-folate toxicity
  • Live vaccines — contraindicated
  • PPIs — reduce MTX clearance

Monitoring

  • FBC and LFTs (2-weekly for first 3 months, then 3-monthly)
  • Renal function
  • Chest symptoms (pneumonitis — rare but serious)
  • Mucositis

Reference: BNFc; BNF; BAD Methotrexate Dermatology Guidelines; MHRA MTX Safety. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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