At the federal level, marijuana remains classified as a Schedule I substance under the Controlled Substances Act, where Schedule I substances are considered to have a high potential for dependency and no accepted medical use, making distribution of marijuana a federal offense. In October of 2009, the Obama Administration sent a memo to federal prosecutors encouraging them not to prosecute people who distribute marijuana for medical purposes in accordance with state law.
In late August 2013, the U.S. Department of Justice announced an update to their marijuana enforcement policy. The statement read that while marijuana remains illegal federally, the USDOJ expects states like Colorado and Washington to create "strong, state-based enforcement efforts.... and will defer the right to challenge their legalization laws at this time." The department also reserves the right to challenge the states at any time they feel it's necessary.
More recently, in January 2018, former Attorney General Sessions issued a Marijuana Enforcement Memorandum that rescinded the Cole Memorandum, and allows federal prosecutors to decide how to prioritize enforcement of federal marijuana laws. Specifically, the Sessions memorandum directs U.S. Attorneys to “weigh all relevant considerations, including federal law enforcement priorities set by the Attorney General, the seriousness of the crime, the deterrent effect of criminal prosecution, and the cumulative impact of particular crimes on the community.” Text of the memo can be found here: https://www.justice.gov/opa/pr/justice-department-issues-memo-marijuana-enforcement
NCSL's policy on state cannabis laws can be found under Additional Resources below.
Arizona and the District of Columbia voters passed initiatives to allow for medical use, only to have them overturned. In 1998, voters in the District of Columbia passed Initiative 59. However, Congress blocked the initiative from becoming law. In 2009, Congress reversed its previous decision, allowing the initiative to become law. The D.C. Council then put Initiative 59 on hold temporarily and unanimously approved modifications to the law.
Before passing Proposition 203 in 2010, Arizona voters originally passed a ballot initiative in 1996. However, the initiative stated that doctors would be allowed to write a "prescription" for marijuana. Since marijuana is still a Schedule I substance, federal law prohibits its prescription, making the initiative invalid. Medical marijuana "prescriptions" are more often called "recommendations" or "referrals" because of the federal prescription prohibition.
States with medical marijuana laws generally have some form of patient registry, which may provide some protection against arrest for possession up to a certain amount of marijuana for personal medicinal use.
Some of the most common policy questions regarding medical marijuana include how to regulate its recommendation, dispensing, and registration of approved patients. Some states and localities without dispensary regulation are experiencing a boom in new businesses, in hopes of being approved before presumably stricter regulations are made. Medical marijuana growers or dispensaries are often called "caregivers" and may be limited to a certain number of plants or products per patient. This issue may also be regulated on a local level, in addition to any state regulation.