Vinblastine and erythromycin: an unrecognized serious drug interaction.

Vinblastine and erythromycin are among the most commonly used chemotherapeutic and antimicrobial agents, respectively. No interaction between the two has ever been reported. Towards the end of a phase I study of vinblastine plus oral cyclosporin (to reverse multidrug resistance), three patients also received erythromycin to raise their cyclosporin levels. All developed severe toxicity consistent with a much higher vinblastine dose than was actually given. This apparent potentiation of vinblastine toxicity has not been previously described.