Methotrexate — SCE Dermatology MCQ

Correct answer: A — On a different day from Methotrexate (typically the day after)

The correct answer is A, on a different day from methotrexate (typically the day after). Methotrexate works as a folate antagonist by inhibiting dihydrofolate reductase, and folic acid supplementation is given to offset adverse effects (nausea, mucositis, macrocytosis, hepatotoxicity) without blunting the antifolate therapeutic effect. UK guidance for methotrexate in psoriasis and other conditions is that folic acid 5 mg once weekly should be taken on a different day to methotrexate, not simultaneously, because same day dosing could theoretically compete with the mechanism of action. This timing separation is standard practice across NHS methotrexate monitoring and patient information, and is what patients are counselled to follow, for example take the folic acid the day after the methotrexate dose. Why the other options are wrong: D. Only when blood tests are abnormal: Folic acid is a routine co-prescription from the outset of methotrexate therapy to reduce toxicity risk, not a reactive measure triggered by abnormal bloods such as macrocytosis or deranged liver function. B. On the same day as methotrexate: This is specifically discouraged because it may reduce methotrexate efficacy by directly opposing its antifolate mechanism at the time of peak drug action. C. Only monthly: The standard regimen is weekly dosing (usually 5 mg once weekly), not monthly, to provide consistent folate replenishment between methotrexate doses. E. It doesn't matter when: Timing is deliberately specified in guidance precisely because same day administration risks antagonising methotrexate's therapeutic effect, so timing is clinically relevant, not arbitrary. Key point: Folic acid should be taken on a day separate from methotrexate, typically the day after, to reduce toxicity while preserving methotrexate's antifolate efficacy.

Reference: NHS.uk, How and when to take methotrexate, 2023 (https://www.nhs.uk/medicines/methotrexate/how-and-when-to-take-methotrexate/); consistent with BNF and NICE CKS DMARD monitoring guidance.