Interaction occurs when spheres of activity of multiple substances overlap in a way that the effect of one substance will influence the behaviour of another. In most cases of interactions between drugs and fruit juices the interaction is of pharmacokinetic nature, meaning that one or more substance from the juice modifies the absorption, distribution, metabolism and/or excretion of the drug. The cytochrome P450 (CYP) group of enzymes is responsible for the metabolism of most xenobiotics, and the ones most involved in drug interactions are CYP1A2, 2C9, 2C19, 2D6, 2E1 and 3A4. When cytochrome inhibition occurs, the concentration of the drug substrate rises. On the contrary, if the substrate is a prodrug, the concentration of its active metabolite and therapeutic effect will reduce. Besides the CYP enzymes, transporters such as P-glycoprotein (P-gp) and organic anion transporter peptide (OATP) often participate in drug-fruit juice interactions. Fruit juice is the most popular fruit product thanks to its nutritious and health-promoting properties. Considering its physical properties and manufacturing processes, fruit juices can be divided into clear, cloudy and puree. The manufacture of clear juice is based on processes that help remove insoluble particles that disenable juice transparency. The goal of cloudy and puree juice production is to prevent sedimentation of insoluble particles and achieve stability and homogeneity. Biologically active substances are naturally present and non-nutritional food ingredients that have a benficial effect on one's health. Flavonoids are the most widespread group of plant polyphenols, the most common in fruits being flavones, flavonols, flavanones, flavanols, anthocyanins and isoflavones. The fruit juices responsible for interacting with drugs covered in this thesis where made from: grapefruit, orange, Seville orange, tangerine, clementine, pomelo, lemon, lime, apple, cranberry, pomegranate, pineapple, blueberry, grape, banana, plum, prune, mango, guava and avocado. Drugs that are most frequently involved in interactions with juices are substrates of CYP3A4 (the most common xenobiotic metabolising enzyme), namely calcium channel blockers, sildenafil, cyclosporine, statins, carbamazepine etc., and most notable fruit juice polyphenols that inhibit CYP3A4 are: bergamottin and 6', 7'-dihydroxybergamottin from grapefruit, Seville orange and pomelo; flavonols, anthocyanins and phenolic acids from cranberry; naringin and naringenin from pomelo; quercetin, kaempherol and gallic acid from pomegranate and finally resveratrol from grape.