Possible Alteration of Metronidazole Metabolism by Phenobarbital

To the Editor: The rare failure of metronidazole to eradicate vaginal trichomoniasis is usually attributed to a resistant organism. We recently encountered a young woman who had recurrent episodes of vaginal trichomoniasis, all of which were documented by wet-mount microscopical examination. Over a period of one year she responded to both conventional single-dose and seven-day courses of metronidazole, but she had symptomatic and microbiologic relapses almost immediately upon discontinuing the medication. Her husband was treated concomitantly on each occasion, and reinfection was not thought to be a problem. The patient was receiving phenobarbital (100 mg daily) for a seizure disorder,.