Infectious mononucleosis-like syndrome induced by salazosulfapyridine in a patient with rheumatoid arthritis.

We describe a 43-year-old woman with rheumatoid arthritis (RA), who developed severe infectious mononucleosis (IM)-like syndrome during treatment with salazosulfapyridine (SASP). She presented with fever, skin rash, lymphadenopathy, and hepatosplenomegaly. Laboratory tests revealed a marked increase of atypical lymphocytes in the peripheral blood and biphasic hepatic dysfunction. IM-like syndrome can be caused by various drugs, including SASP, and the concept of drug-induced hypersensitivity syndrome has been proposed recently. IM-like syndrome due to SASP has been reported in patients taking higher dosages for the treatment of inflammatory bowel disease, but has not been reported earlier in patients with RA. The results of the drug-induced lymphocyte stimulation test tests suggested that 5-aminosalicylic acid was a possible causative metabolite. This severe type of drug-induced hypersensitivity reaction mimicking IM due to SASP should be granted wider awareness in the field of rheumatology, because the drug is widely used for the treatment of RA.