Most people get information about marijuana from friends, the Internet, newscasts and personal experience – all echo chambers filled with anecdotes, opinions, and little science. Clinicians receive little education about marijuana. From Bud to Brain provides health professionals the science of marijuana needed to offer the public objective and relevant advice about the safe and effective use of marijuana. The need is huge: 1 out of 5 US citizens can buy recreational marijuana legally and over 200 million have access to medical marijuana. Products from the cannabis plant include dried buds (marijuana), resin (hashish) and concentrates (dabs, budder) that can be smoked, vaped or fashioned into edibles. Understanding how THC produces the experience of being high requires understanding the brain’s natural THC-like chemistry and what parts of the brain are impacted by marijuana. Tracing the research discoveries leading to understanding the science of marijuana gives clinicians the scientific context to help patients make wise decisions about its use. The principles of motivational interviewing are reviewed to help clinicians communicate a science-based perspective on marijuana to recreational users, medical patients, adolescents, worried parents and heavy users.