Tucked deep inside the 1,603-page federal spending measure is a provision that effectively ends the federal government’s prohibition on medical marijuana and signals a major shift in drug policy.

The bill’s passage over the weekend marks the first time Congress has approved nationally significant legislation backed by legalization advocates. It brings almost to a close two decades of tension between the states and Washington over medical use of marijuana.

Under the provision, states where medical pot is legal would no longer need to worry about federal drug agents raiding retail operations. Agents would be prohibited from doing so.

More: Congress Quietly Ends Federal Government’s Ban on Medical Marijuana