The stimulus properties of two common over-the-counter drug mixtures: dextromethorphan + ephedrine and dextromethorphan + diphenhydramine

Two groups of rats were trained in a two-choice drug discrimination procedure under a fixed-ratio 10 schedule of food reinforcement. One group of rats (n=12) was trained to discriminate the presence and absence of a drug mixture containing 10 mg/kg dextromethorphan + 10 mg/kg diphenhydramine. The other group of rats (n=12) was trained to discriminate the presence and absence of another drug mixture containing 10 mg/kg dextromethorphan + 10 mg/kg ephedrine. Cross-generalization tests conducted with each of the stimulus elements demonstrated that (1) the drug mixtures were not perceived as new entities distinct from their component elements and (2) the stimulus element saliency may be a factor determining the nature of discriminative control by drug mixtures. Cross-generalization tests conducted with the psychomotor stimulants, cocaine and amphetamine, engendered complete generalization to the training cues in both groups, whereas, pentobarbital engendered predominantly saline- or default-lever responding. These data suggest a potential abuse liability for both of these common over-the-counter drug mixtures and cautions against the use of such combinations in pediatric patients.