US President Donald Trump’s acting attorney general on Thursday signed an order reclassifying state-licensed medical marijuana as a less dangerous drug, a major policy shift long sought by advocates who said cannabis should never have been treated like heroin by the federal government.

The order signed by Todd Blanche does not legalise marijuana for medical or recreational use under US law. But it does change the way it is regulated, shifting licensed medical marijuana from Schedule I – reserved for drugs without medical use and with high potential for abuse – to the less strictly regulated Schedule III. It also gives licensed medical marijuana operators a major tax break and eases some barriers to researching cannabis.

The Trump administration also said it was jump-starting the process for reclassifying marijuana more broadly, setting a hearing to begin in late June.

Trump told his administration in December to work as quickly as possible to reclassify marijuana. On Saturday, as the Republican president signed an unrelated executive order about psychedelics, he seemed to express frustration that it was taking so long.

Blanche said on Thursday that the Department of Justice was “delivering on President Trump’s promise” to expand Americans’ access to medical treatment options. “This rescheduling action allows for research on the safety and efficacy of this substance, ultimately providing patients with better care and doctors with more reliable information,” he said in a statement.

Blanche’s action largely legitimises medical marijuana programmes in the 40 states that have adopted them. It sets up an expedited system for state-licensed medical marijuana producers and distributors to register with the US Drug Enforcement Administration.