Fixed drug eruption due to metronidazole.

A 37-year-old woman was examined for a large, dark brown plaque on her left hip that had been present for three years and a second, similar plaque that had subsequently appeared on her right knee. Every four to five months the areas became swollen, red, and painful. She had been taking metronidazole intermittently for twenty years for the treatment of trichomoniasis; this drug on challenge proved to be the cause of the eruptions.