Alcohol interacts with a number of drugs in a paradoxical way; when alcohol and drugs are coingested, effect of drug on the body appears to be prolonged and enhanced, not only by additive process at the target organ but also by means of impaired absorption and metabolism of drug (= drug tolerance). By contrast, chronic alcoholics, who have proliferated endoplasmic reticulum and adaptively induced drug-metabolizing enzymes in the liver, when sober, require larger doses of drug than usually in order to produce a proper effect (= drug tolerance). The induction of drug-detoxifying enzymes by alcohol, however, could result in the enhancement of the hepatotoxic effect of some chemicals such as carbon tetrachloride. Clinical importance of alcohol-drug interactions is such that physician must be cautious about the ingestion of alcohol by his patients taking other medications and that he should rely heavily on careful patient observation to determine efficacy or toxicity of drug prescribed for alcoholic patients.