Quinidine markedly increased the
plasma levels of
dextromethorphan in one study. Some of the patients (given
dextromethorphan 60 mg) experienced dextromethor- phan
toxicity (nervousness, tremors, restlessness, dizziness, shortness of breath, confusion etc.).
It seems likely that at normal therapeutic doses of
dextromethorphan will cause a problem in most patients taking quinidine, but be aware that some may become more sensitive to its effects.