Cannabis has been used therapeutically for more than three thousand years. Until 1937 cannabis was part of the accepted pharmacopeia in the United States. After the “reefer madness” mania of the late 1930s cannabis was made illegal under the Tax Cannabis Act of 1937. 
It’s interesting to note that the only major organization to protest the Cannabis Act was the American Medical Association. That’s because their physicians knew first-hand the beneficial properties of cannabis.
Cannabis remains illegal today at the Federal level. However, in California and a number of other states cannabis has been declared legal for medical purposes. California Proposition 215 provides a wide measure of latitude for legitimate medical users of cannabis.