Common Registration Assessment, Part 1
Methotrexate weekly dose calculations
Three original questions on once-weekly methotrexate doses and tablet counts, with the working shown.
Published 14 October 2026. Independent revision material, not GPhC questions, not for patient care.
Methotrexate for autoimmune disease is taken once a week. The MHRA Drug Safety Update of September 2020 reports serious toxicity from daily dosing by mistake, and advises dispensers to remind the patient of weekly dosing and to write the day of the week on the pack where there is space.
The emc SmPC for methotrexate 2.5 mg tablets gives an initial dose in rheumatoid arthritis of 7.5 mg once weekly, not to exceed 20 mg a week, and 10 to 25 mg once weekly for severe psoriasis. These are non-cancer uses only. The BNF methotrexate entry still carries the 2020 MHRA advice and recommends that only one strength of tablet, usually 2.5 mg, is prescribed and dispensed. The Pharmaceutical Journal reported on 19 December 2025 that the GPhC reminded pharmacy teams to supply methotrexate in line with that 2020 update. The calculations here are tablet counts and weekly totals.
The method
- Write the weekly dose in mg and the tablet strength in mg.
- Divide the dose by the tablet strength to get tablets per week.
- Multiply by the number of weeks for a supply, or divide the pack size by tablets per week to find how many weeks a pack lasts.
- To size a daily dosing error, multiply the dose given each day by 7 and compare with the weekly dose.
- State the day of the week on the label, never a daily instruction.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A patient has methotrexate 7.5 mg once weekly using 2.5 mg tablets. How many tablets is one dose, and how many for 4 weeks?
Show the answer
Answer: 3 tablets per dose, 12 tablets for 4 weeks
Working: 7.5 ÷ 2.5 = 3 tablets once a week. 3 × 4 = 12 tablets.
Question 2
The same patient takes 3 tablets every day for a week by mistake. How many mg is that, how many times the intended weekly dose, and what should the patient be told to do?
Show the answer
Answer: 52.5 mg, which is 7 times the intended dose. This is a serious medication error: the patient should seek urgent medical attention.
Working: Per day: 7.5 mg. Over 7 days: 7.5 × 7 = 52.5 mg. 52.5 ÷ 7.5 = 7 times. The MHRA Drug Safety Update says patients should seek urgent medical attention if they think they have taken too much methotrexate. The arithmetic only sizes the error; the clinical response comes from a prescriber or emergency service, not from this page.
Question 3
A patient has 10 mg once weekly using 2.5 mg tablets, supplied in a pack of 28 tablets. How many tablets per dose, and how many weeks does a pack last?
Show the answer
Answer: 4 tablets per dose, and 7 weeks
Working: 10 ÷ 2.5 = 4 tablets a week. 28 ÷ 4 = 7 weeks.
Exam approach
Convert the weekly dose to tablets, then scale by weeks. For an error, multiply the dose by the number of days taken.
Where marks are lost
- Reading 'once weekly' as a daily dose.
- Mixing up 2.5 mg and 10 mg tablets when converting mg to tablets.
- Forgetting that the maximum weekly dose in the SmPC for rheumatoid arthritis is 20 mg.
- Leaving the day of the week off the label.
- Treating a daily dosing error as an arithmetic exercise only: the patient needs urgent medical attention.
Frequently asked questions
What did the MHRA advise dispensers about once-weekly methotrexate?
The Drug Safety Update says to remind the patient of the once-weekly dosing and the risk of fatal overdose if they take more than directed, and to write the day of the week on the outer package where there is space.
How many 2.5 mg tablets make 20 mg?
20 ÷ 2.5 = 8 tablets. The SmPC says the weekly total in rheumatoid arthritis should not exceed 20 mg.
Sources
- MHRA, methotrexate once-weekly for autoimmune diseases (Drug Safety Update, 23 September 2020)
- emc, methotrexate 2.5 mg tablets SmPC (last updated 16 September 2026)
- BNF, methotrexate
- The Pharmaceutical Journal, GPhC writes to pharmacy teams after methotrexate dispensed with instruction to take once daily (19 December 2025)
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
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