Dimenhydrinate is an over-the-counter drug used to prevent nausea and motion sickness. It is most commonly prepared as tablets, although it is also available in liquid form and in suppositories. Dimenhydrinate is a combination of two drugs: diphenhydramine and 8-chlorotheophylline, a chlorinated derivative of theophylline. Diphenhydramine is the primary constituent of dimenhydrinate and dictates the primary effect. The main difference relative to pure diphenhydramine is a lower potency due to being combined with 8-chlorotheophylline. 8-Chlorotheophylline was added in order to counteract drowsiness. Theophylline is very closely related to caffeine and theobromine, mild central nervous system stimulants. It was thought by scientists that by combining the antiemetic effects of diphenhydramine with a stimulant, the extreme drowsiness induced by the former could be mitigated somewhat by the latter.
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