Co-existing sensitivity to metronidazole and isothiazolinone.

A 68-year-old woman developed allergic contact dermatitis to topical metronidazole gel as proven by positive patch tests to the gel and to metronidazole. She was also allergic to methylchloroisothiazolinone and methylisothiazolinone (MC/MI). The similarity between the two molecules and the fact that the patient reacted to the gel after the very short incubation period of 1 day (i.e. not long enough for acquiring an active sensitization) makes the possibility of a cross-reaction between these substances very plausible. As the isothiazolinones are widely used and comprise an important and relatively frequent cause of allergic contact dermatitis, a cross-reactivity with metronidazole means that perhaps there should have been more cases of metronidazole allergy is more common than the current literature suggests.