Acute behavioral and cardiac effects of alcohol and caffeine, alone and in combination, in humans

The acute behavioral and cardiac effects of alcohol (0, 0.5 and 1.0 g/kg) and caffeine (0, 250 and 500 mg/70 kg), administered alone and in combination, were assessed in eight adult humans. Subjects received all possible combinations twice. Alcohol administered alone disrupted responding in the Digit-Symbol Substitution Test and the Repeated Acquisition and Performance Procedure, increased heart rate, decreased blood pressure, and increased subject ratings of drunkenness. Caffeine administered alone offset performance decrements that emerged across the test session on the Digit-Symbol Substitution Test performance and accuracy of responding in the Repeated Acquisition and Performance Procedure, but never actually enhanced performance and learning. Caffeine administered alone increased blood pressure and increased subjective ratings of drug strength. The most notable effect of the drug combinations was that caffeine partially attenuated the disruptive behavioral effects of alcohol. Combining alcohol and caffeine generally offset the pressor effects observed with the drugs administered alone. By contrast, alcohol-caffeine combinations did not significantly alter breath alcohol levels, heart rate or subject-rated drug effects, relative to the effects of the drugs alone. Across all measures except heart rate, these effects are qualitatively similar to those observed previously with cocaine and d-amphetamine in combination with alcohol, documenting a high degree of consistency in the behavioral pharmacology of alcohol-stimulant combinations in humans.