Three senators are introducing a bill to remove federal prohibitions on medical marijuana in states where it is already legal. The measure is being unveiled on Capitol Hill today by Republican Rand Paul of Kentucky and Democrats Kirsten Gillibrand of New York and Cory Booker of New Jersey. It will aim to eliminate the uncertainty surrounding use of marijuana in the states that allow it for medicinal purposes. Twenty-three states and the District of Columbia have such laws. Advocates say that even in those states, the federal prohibition on marijuana prevents doctors at veterans’ facilities from prescribing it for medical purposes and makes access difficult. Marijuana legislation has been introduced in the House. Advocates say the introduction of a high-profile Senate bill is a sign of growing acceptance.