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Background Ethanol and caffeine are the most widely used psychoactive substances in the world, with an observed steady increase in the combined consumption of alcohol and caffeine. Specific signs of ethanol-caffeine interactions have been reported both in humans and in animals. The metabolic effects of these interactions have not been fully elucidated. There are no published reports on the influence of caffeine on ethyl glucuronide (EtG) formation. EtG is a direct metabolite of ethanol and is very often used as a biomarker of alcohol consumption. Here, we investigated the influence of caffeine on the formation of EtG in rat plasma and EtG incorporation into the hair. Methods Studies were conducted on three male Wistar rat groups, each receiving either ethanol at 3 g/kg/day, ethanol (at the same dose) with caffeine at 3 mg/kg/day, or caffeine at 3 mg/kg/day for four weeks. EtG and caffeine levels were evaluated in hair and in blood after the last administration. Results Blood EtG levels after the administration of ethanol together with caffeine were significantly higher than after the administration of ethanol alone. EtG levels in rat hair in the ethanol-and-caffeine group were also higher than in the ethanol-only group, but the difference was not statistically significant. Conclusion This study shows the possible effect of ethanol and caffeine co-administration on EtG formation. Caffeine stimulates EtG synthesis resulting in increased blood and, possibly, hair levels of this metabolite. However, the role of these changes in estimating alcohol consumption requires further studies.

The effect of quinidine (QD) and grapefruit juice (GFJ) extract, P-glycoprotein inhibitors, on the domperidone (DOM) concentration in rat plasma was investigated. DOM, a dopamine D(2)-receptor antagonist is a substrate for P-glycoprotein. DOM(10 mg/kg) was administered orally 2 h after GFJ extract (0.2 ml/kg) or QD (25 mg/kg). DOM concentration in plasma samples was determined by HPLC with fluorescence detection. The GFJ extract and QD administration significantly increased c(max) of DOM by 19% and 36%, respectively, and the AUC(0-0.25) (area under the concentration-time curve from time zero to 15 min) by 29% and 44%, respectively. In addition, QD significantly increased the DOM AUC(0-2) (32%), whereas 19% increase was observed after GFJ extract administration. In conclusion, GFJ and QD significantly influenced DOM rat plasma concentration during the first two hours after DOM administration indicating that interaction takes place during absorption phase.