Methotrexate (Low-dose)

Source: UK SPC (eMC) §4.2 for Methotrexate 10 mg Tablets (https://www.medicines.org.uk/emc/product/11723/smpc). VERBATIM RA REGIMEN: 'The usual dose is 7.5 - 15 mg once weekly. The schedule may be adjusted gradually to achieve an optimal response but should not exceed a total weekly dose of 20 mg. Thereafter the dose should be reduced to the lowest possible effective dose which in most cases is achieved within 6 weeks.' ONCE-WEEKLY SAFETY WARNING (§4.2/§4.4): 'In the treatment of rheumatoid arthritis and psoriasis, methotrexate must only be taken once a week.' Dosage errors can result in serious adverse reactions, including death; mistaken daily use of the recommended dose has led to fatal toxicity. The prescriber must ensure the patient or carer can comply with the once-weekly regimen and must specify the day of intake. Methotrexate should only be prescribed by physicians with expertise in its use and a full understanding of the risks. PSORIASIS (other indication in the same SPC): give a test dose of 2.5-5.0 mg first to exclude unexpected toxic effects; if laboratory tests are normal one week later, treatment may be initiated at the usual dose of 7.5-15 mg once weekly, increased as necessary up to a total weekly dose of 25 mg, then reduced to the lowest effective dose (response usually achieved within 4 to 8 weeks); check liver function before starting and repeat at 2 to 4 month intervals. ELDERLY: use with extreme caution; a dose reduction should be considered because of reduced liver and kidney function and lower folate reserves with increasing age. HEPATIC IMPAIRMENT: administer with great caution, if at all, to patients with significant current or previous liver disease, especially if alcohol-related; if hepatic function abnormalities develop, suspend dosing for at least two weeks. THIRD DISTRIBUTION SPACE (pleural effusions, ascites): half-life can be prolonged to 4 times normal — dose reduction or, in some cases, discontinuation may be required. ROUTE SWITCH: if changing from oral to parenteral administration a dose reduction may be required because of the variable bioavailability of oral methotrexate. PAEDIATRIC: no paediatric dose is given in this UK SPC. The US label (DailyMed, Alembic, 2026-05-18) states a recommended STARTING dosage for polyarticular juvenile idiopathic arthritis of 10 mg/m2 orally once weekly, adjusted to optimal response — this is a body-surface-area dose, not a per-kg dose, so paedDose is null; verify any under-18 use against a children's formulary. §4.5 was not retrieved in this bundle (§4.4 truncated at the source-fetch limit) — the interactions listed below are from the US label §7 plus the §4.3 antifolate/live-vaccine entries; verify against the full UK §4.5.