The objectives of this study are to carefully describe the context of methotrexate medication errors, to details medical consequences and management approaches, and to determine the rate of fatal outcome.
Data on methotrexate medication errors were obtained from the French network of poison control and pharmacovigilance centres, which collected and documented reported drug-induced adverse effects. Cases were included if the intake was more than 2-fold the intended weekly dose or a weekly cumulative dose ≥ 30 mg and a follow-up of at least 4 days after the last dose. Data were analysed for demographics, treatment indication, prescribed dose, drug interactions, clinical complications and medical outcomes.
Seventy four patients were included. The causes of methotrexate errors resulted from an erroneous prescription renewal (23.3%), incomprehensiveness of the weekly schedule by patients or at-home caregivers (56.2%) and administration of a wrong dose by a health care professional (20.5%). Of the 70 patients who took methotrexate daily, the mean daily dose received over the whole duration of the error was 9.6 ± 4.1 mg (range 2.5-22.5) with a mean duration of the error of 11.7 ± 12.2 days (range 2 to 90). Thirteen (18%) patients remained asymptomatic and 61 (82%) developed complications of which 46 (62.2%) were severe. Nine (14.8%) patients died within 11 to 45 days after the first dosing error. Compared to patients with no or mild symptoms, those with severe symptoms were more likely to be older (75.6 ± 10.8 vs. 69.5 ± 12.9 years) and to be exposed to a higher cumulative dose (94.8 ± 46.2 vs. 68.0 ± 45.7 mg).
This study confirms that dosing errors with methotrexate can be lethal and persisted despite several warnings from drug agencies. Further measures are awaited from the European Medicine Agency.