Delayed reactivation of haloperidol induced photosensitive dermatitis by methotrexate.

A man whose psoriasis was well controlled on methotrexate treatment developed pellagra-like photosensitive dermatitis when he started taking haloperidol. Reactivation of his dermatitis was observed with methotrexate as a recall photosensitivity phenomenon. This false photosensitivity reaction of methotrexate is an important but rarely encountered adverse effect. A possible interaction between methotrexate and haloperidol is emphasised.