Doxylamine: a cause for false-positive gas chromatographic assay for phencyclidine.

A 25-year-old white woman ingested an unknown quantity of doxylamine succinate and flurazepam. Urine immunoassay screen (EMIT-dau) was positive for benzodiazopine and negative for phencyclidine. Subsequent gas chromatographic assay of the urine revealed a markedly positive assay for phencyclidine. Doxylamine was ultimately found to be the cause for the false-positive gas chromatographic assay for phencyclidine.