Case study: Accidental overdose

...Mr K presents with nausea and vomiting and it transpires he had been mistakenly following his previous medication routine...

  • Date: 17 December 2020


Mr K is 59 years old and has been referred from his GP practice to a consultant rheumatologist for pain affecting both hands and forearms. He is diagnosed with bilateral carpal tunnel syndrome with underlying rheumatoid arthritis. He is prescribed methotrexate 15mg for two weeks and then 20mg taken weekly in eight 2.5mg tablets. He undergoes carpal tunnel decompression in both limbs.

Later the GP practice receives a form letter from the hospital stating that Mr K has been prescribed methotrexate “20mg weekly”. A GP – Dr Q – issues a repeat prescription for methotrexate 20mg weekly in 10mg tablets to be taken twice weekly. A week later Mr K presents at the surgery with nausea and vomiting and on review is found to have overdosed on methotrexate, having taken 80mg instead of the 20mg prescribed. This was because he had been following his previous routine of eight tablets weekly.

The treating GP – Dr L – consults the local poisons unit and bloods are taken to check methotrexate levels. Treatment is suspended and the consultant rheumatologist writes to the practice advising weekly blood tests and resumption of methotrexate after one month if blood levels are okay.

Six months later a letter of claim is sent to Dr Q alleging clinical negligence in his treatment of Mr K – specifically in prescribing the incorrect dosage of methotrexate tablets. It is claimed that the error lead to a seven-week period in which the patient was unable to take methotrexate to treat his rheumatoid arthritis. In this period he suffered significant joint pain and effusions in both knees requiring aspiration.


MDDUS lawyers assess the case and commission an expert GP review. The expert observes that the letter sent by the hospital states that the patient was to be prescribed methotrexate 20mg weekly. No further information was provided with regard to the maintenance dose. The expert thus opines that it was not unreasonable for Dr Q to issue a prescription of 10mg twice weekly and that the methotrexate overdose appears to have been caused by an error made by the patient. It is acknowledged that good practice guidance on methotrexate dosage has subsequently changed with the default dose now being 2.5mg tablets – but this was not the case at the time of the incident.

MDDUS draws up a letter of response on behalf of Dr Q denying liability and the claim is subsequently dropped.


  • Ensure where possible the patient is clear on dosage, especially if there is any uncertainty in primary to secondary handover.
  • Take particular care prescribing medications such as methotrexate, which can be potentially toxic if given in the wrong dose.

This page was correct at the time of publication. Any guidance is intended as general guidance for members only. If you are a member and need specific advice relating to your own circumstances, please contact one of our advisers.

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Insight Primary is published quarterly and distributed to MDDUS members throughout the UK who work in primary care. It provides a mix of articles on risk, medico-legal and regulatory matters as well as general features and profiles of interest to our members.
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