The amendment constitutes a single paragraph of federal law. It prohibits the Justice Department from spending even a cent to prosecute medical marijuana users and sellers operating legally under state laws.
Since its passage, it has largely shut down efforts by federal prosecutors or drug enforcement officials to interfere with otherwise legal sales of marijuana in 29 states and the District of Columbia that have passed legalization measures.
The prospect that the ban on prosecutions could expire has spread anxiety across the marijuana industry.
The uncertain fate of the pot provision has created tension among Republicans, dozens of whom have cast votes to prevent the federal government from a crackdown on medical marijuana. Many would like to do so again.