Murphy's law in force: sequential adverse events encountered during the treatment of Pneumocystis pneumonia (cotrimoxazole-induced acute peripheral neuropathy and primaquine-induced methemoglobinemia).

Methotrexate monotherapy is a common management strategy in rheumatoid arthritis (RA). Treatment with immunosuppression can lead to opportunistic infections such as Pneumocystis jirovecii pneumonia (PJP). The treatment options for PJP include cotrimoxazole, clindamycin-primaquine and dapsone. Though these drugs are generally well tolerated, they can result in potentially severe adverse effects. Sometimes several undesired events may occur in a single patient, reminding us of Murphy's law. Herein, we report a case which exemplifies this adage. A 50-year-old female developed PJP, while on methotrexate therapy for RA and was treated with cotrimoxazole. The latter resulted in painful peripheral neuropathy, which improved after cotrimoxazole was stopped. Salvage therapy for PJP with primaquine-clindamycin, lead to another serious adverse event, methemoglobinemia. Withdrawing the offending drug resulted in dramatic improvement.