Cannabis sativa, or marijuana, has been used medicinally for millennia. In the United States, it was part of the published pharmacopeia until 1942, and was still legally available for medical use until 1970, when the Controlled Substances Act was passed. Thereafter, marijuana was classified as a schedule I controlled substance, with a high potential for abuse and no medically recognized use. For 26 years after this law was passed, with limited exceptions, there was no medical role for marijuana in the United States.

Since the Compassionate Use Act of 1996 was passed by popular vote in California, several states have sought to make marijuana available for medical use to qualifying patients. To date, 13 states have voted by popular referendum or passed laws to provide a substantive degree of legal sanction for the medical use of marijuana, the most recent being Michigan in 2008 (Table 1). In only one instance, in South Dakota in 2006, has a state referendum on this issue failed to pass with a majority of the votes cast.

Nearly 25 percent of the nation's population lives in the 13 states where the use of marijuana for medicinal purposes is allowed: Alaska, California, Colorado, Hawaii, Maine, Michigan, Montana, Nevada, New Mexico, Oregon, Rhode Island, Vermont, and Washington. In California alone, it is estimated that physicians have recommended marijuana to as many as 350,000 patients.¹ Another study estimated that 1,000 to 2,000 licensed physicians have authorized the use of medical marijuana for patients in Washington.² The American Academy of Family Physicians “opposes the use of marijuana except under medical supervision and control for specific medical indications.”³

Medicolegal and political issues tend to overshadow the science and the medicine of marijuana use. It should be noted that although some states have tried to liberalize medical access to marijuana, the federal government remains firm in its stance: marijuana is a schedule I controlled substance and is monitored by the U.S. Drug Enforcement Administration (DEA). The U.S. Food and Drug Administration has issued statements decidedly opposed to the medical use of marijuana.⁴ In the 2005 decision Gonzales v. Raich, the U.S. Supreme Court upheld the right of the federal government to maintain the schedule I classification. On the other hand, the court has also implicitly maintained that state initiatives on marijuana use should not be preempted by federal law. Recently, U.S. Attorney General Eric Holder, Jr., said that the DEA will not raid dispensaries of marijuana. It is anticipated that the Obama administration will look more favorably on the issue of medical marijuana than previous administrations have.

Studies on marijuana used medicinally as opposed to recreationally are lacking; this is, at least in part, because of the current scheduling of the drug. However, the available evidence shows that marijuana is effective in the treatment of chronic pain, spasticity resulting from various diseases (including multiple sclerosis), severe nausea, anorexia and wasting, glaucoma, and seizure disorders.^5,6 Each state that allows the medical use of marijuana has ratified its own set of approved indications ( Online Table A). There is reasonable concern about the safety profile, and some evidence that significant adverse effects must be taken into account. These include acute cardiovascular and psychogenic effects, such as anxiety, as well as long-term effects related to smoking, such as chronic lung disease and cancers.^7-10 The issue of whether marijuana smoke is less carcinogenic than cigarette smoke is debatable.¹¹

Resources on the medical uses of marijuana are listed in Online Table B. These include research and science, where they exist, into the subject, as well as information about individual states' approaches to the use of marijuana, ranging from indications for which it is approved to details about how patients can obtain it and how much they may possess. Physicians should become familiar with their own state's laws, because the details for how marijuana can be recommended vary from state to state.