BOSTON — Several U.S. senators are urging federal regulators to make it clear that partially filling prescriptions for opioid drugs is permitted under federal law.

Massachusetts Sens. Elizabeth Warren and Edward Markey said in a letter to the Drug Enforcement Administration that officials in many states — including Massachusetts — have pushed for the change as a way to combat the opioid addiction problem.

They said "partial-fill" policies let patients work with their doctors and pharmacists to control pain while limiting the amount of unused medication in their home.

They said while federal law doesn't specifically ban the practice, the lack of clarity makes some states hesitant.

The letter was also signed by fellow Democratic Sen. Richard Blumenthal of Connecticut and Republican Sens. Shelley Moore Capito of West Virginia and Kelly Ayotte of New Hampshire.