Ethanol as a Modifier of Drug Toxicity in Humans: Pathways of Toxicity and Organ-Level Consequences

Ethanol consumption can modify both drug exposure and drug response. However, the clinical relevance of these interactions depends strongly on the timing and pattern of alcohol intake, the affected pharmacological pathway, the dosage form and organ reserve. This review summarizes current evidence on ethanol-drug interactions, particularly human crossover studies, phenotyping studies, cohort analyses and appropriate case reports. It distinguishes acute ethanol-drug co-exposure, chronic alcohol exposure, drug use during early abstinence after chronic drinking, and pharmacotherapy in alcohol-associated liver disease. Key mechanisms include ADH- and ALDH-dependent ethanol oxidation, acetaldehyde formation, NADH/NAD⁺ redox shift, CYP2E1 induction, carboxylesterase 1 (CES1) modulation, altered intestinal and hepatic first-pass handling, dose dumping from susceptible modified-release products, changes in protein binding in alcohol-associated liver disease, and ALDH inhibition with acetaldehyde accumulation in disulfiram-like reactions. At the molecular level, ethanol may promote acetaldehyde adduct formation with proteins and DNA, CYP2E1-driven reactive oxygen species generation, redox stress, intestinal barrier injury, and CES1-dependent transesterification of selected ester drugs. Acute ethanol intake mainly increases pharmacodynamic toxicity and causes short-term pharmacokinetic disturbances, including enhanced central nervous system depression, delayed gastric emptying, impaired glucose and lactate handling and altered hemodynamic responses. In contrast, chronic exposure, early abstinence and alcohol-associated liver disease are more often associated with hepatic enzyme and transporter remodeling, altered protein binding, reduced hepatic or renal reserve, and greater susceptibility to drug-related organ injury. The highest-risk scenarios involve older adults, polypharmacy, alcohol-associated liver disease, dehydration or acute illness, early abstinence, and the concurrent use of central nervous system depressants, glucose-lowering drugs, NSAIDs, antihypertensives, renally eliminated drugs or warfarin. Hence, ethanol exposure should be treated as a dynamic, context-dependent modifier factor that can acutely exacerbate pharmacodynamic toxicity, alter selected pharmacokinetic pathways and lower organ tolerance to drug-related injury.