Medical necessity is a legal doctrine in the United States related to activities that may be justified as reasonable, necessary, and/or appropriate based on evidence-based clinical standards of care.
[1] By statute, Medicare may pay only for items and services that are "reasonable and necessary for the diagnosis or treatment of illness or injury or to improve the functioning of a malformed body member" unless there is another statutory authorization for payment.
Cannabis is a plant whose active ingredients are widely reported by patients to be effective in pain control for various conditions, usually neuropathic in nature, in which common painkillers have not had great benefit.
However, the US Supreme Court outrightly rejected that defense in the landmark case United States v. Oakland Cannabis Buyers' Cooperative (2001) which ruled that there is no medical necessity exception to drug laws and that the federal government is free to raid, arrest, prosecute, and imprison patients who are using medical marijuana no matter if the medicine is crucially necessary to them.
In Maryland, a bill signed by Governor Robert Ehrlich became law in 2003 to permit patients to use medical necessity defense to marijuana possession in the state.