Marijuana (cannabis) is the most widely used recreational drug after nicotine and alcohol. This text reviews the rapidly growing body of scientific evidence on cannabis - how it works, the role of endogenous cannabinoids in the brain and body, and the opportunities for new medicines based on such knowledge. The regulation of cannabis use varies considerably throughout the world. This work reviews the rapid changes that have occurred in liberalizing the medical use of cannabis, and in some instances the full legalization of cannabis use, with an emphasis on events in the United States and Canada. The advantages and disadvantages of liberalizing the medical and recreational use of cannabis are discussed in a neutral manner, leaving the reader to decide.