Book excerpt: The trend toward liberalizing medical and recreational marijuana use is increasing the obligation on clinicians to provide useful information to the public. This book summarizes the science all healthcare professionals need to know in order to provide objective and relevant information to a variety of patients, from recreational and medicinal users to those who use regularly, and to adolescents and worried parents. The author brings two and a half decades of studying cannabinoid research, and over forty years' experience in psychiatric and addiction medicine practice, to shed light on the interaction between marijuana and the brain. Topics range from how marijuana produces pleasurable sensations, relaxation and novelty (the 'high'), to emerging medical uses, effects of regular use, addiction, and policy. Principles of motivational interviewing are outlined to help clinicians engage patients in meaningful, non-judgmental conversations about their experiences with marijuana. An invaluable guide for physicians, nurses, psychologists, therapists, and counsellors.