Effects of L-Tryptophan on the Development of Tolerance to the Antitussive Effects of Dihydrocodeine

The effects of L-tryptophan on the development of tolerance to the antitussive effects of dihydrocodeine were examined in rats. Chronic co-administration of L-tryptophan with dihydrocodeine prevented the development of tolerance to the antitussive effects of dihydrocodeine. Furthermore, the antitussive effects of dihydrocodeine in dihydrocodeine-tolerant rats were fully restored by acute co-administration of L-tryptophan, as evidenced by a decrease of about 50% in the ED50 value of dihydrocodeine. Thus, it is concluded that L-tryptophan may antagonize the development of tolerance to the antitussive effects of dihydrocodeine.