© 2017 Bentham Science Publishers. Background & Objective: Cannabis is the most widely used illicit drug. The two most important natural cannabinoids are delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD). The THC content of cannabis has been increasing during the last years and recently appeared in the market as a series of synthetic cannabinoids with potent agonist activity. Recreational users frequently combine cannabis with other drugs of abuse as alcohol, amphetamines and derivatives, nicotine and cocaine. In addition, these subjects can be taking medicines for acute and chronic medical conditions. The increasing use of medicinal cannabis for chronic pain and neurological and psychiatric disorders can produce potential interactions with medications used for the symptomatic treatment of these or other diseases. Conclusion: THC and CBD are metabolized mainly in the liver by cytochrome P-450 isoenzymes (mainly CYP2Cs and CYP3A4). In vitro studies indicate that THC and CBD both inhibit CYP1A1, 1A2 and 1B1 enzymes, and recent studies have indicated that CBD is also a potent inhibitor of CYP2C19 and CYP3A4. Both cannabinoids may interact with other medications metabolized by the same pathway or by inducers/inhibitors of the isoenzymes. Cannabis produces sedation, impairs psychomotor performance, and increases blood pressure and heart rate. Pharmacodynamic interactions with other sedatives can potentiate the central effects but can be decreased by psychostimulants. This review focuses on the interactions between cannabinoids and alcohol, other drugs of abuse, and prescription medicines.