President Donald Trump has signed an executive order that will expand access to cannabis, a long-anticipated move that would mark the most significant shift in US drug policy in decades.

The order directs the U.S. Attorney General to reclassify cannabis from a Schedule I narcotic to a Schedule III drug, placing it under the same category as Tylenol with codeine.

Cannabis will remain illegal at the federal level. But classifying it as a Schedule III narcotic would allow expanded research to be conducted into its potential benefits.

Several Republican lawmakers cautioned against the move, with some arguing it could normalise cannabis use.

The US Drug Enforcement Agency notes that Schedule III narcotics - which also include ketamine and anabolic steroids - have only a "moderate to low potential for physical and psychological dependence".

Speaking in the Oval Office during a signing ceremony on Thursday, Trump said that people have been "begging" for the change, particularly those in "great pain" from ailments including "incurable pain", cancer, seizure disorders and veterans with service-related injuries.

The president compared cannabis to prescription painkillers that have legitimate uses "but can also do irreversible damages".

The new classification also has tax implications for state-authorised cannabis dispensaries, as current regulations bar them from some tax deductions if they sell Schedule I products.

In addition to the rescheduling of cannabis, Trump has ordered White House officials to work with Congress to allow some Americans access to cannabidiol, commonly known as CBD.

Trump has also asked White House officials to work with Congress to allow some Americans access to cannabidiol.

Health officials have also been tasked with developing "methods and models" to examine the real-world health benefits and risks of CBD.

A senior administration official said that the order was a "commonsense action that will let us better understand and study" cannabis and CBD.

In recent years, a majority of US states have approved cannabis for some medical use, and nearly half - 24 - have legalised recreational use. But since 1971, cannabis has been a Schedule I narcotic, which means it has no accepted medical use and a high potential to be abused.

The Biden administration proposed a similar reclassification, and in April 2024 the DEA proposed a rule change, but got bogged down under administrative and legal issues.

Trump's order drew praise from organisations that have been working to change US cannabis regulations.

Tim Barash, chairman of the Coalition of Cannabis Scheduling Reform, said in a statement that the move represents a "fundamental shift" in government and social perceptions of the drug.

"This change will empower the 425,000 people working in the US cannabis industry and bring in new talent, capital, and awareness to an industry that has a positive impact on millions of people's lives," Mr Barash said.

But the reclassification order has met some resistance from Republican lawmakers.

On Wednesday, a group of 22 Republican Senators sent an open letter to the president, arguing that marijuana use would mean that "we cannot re-industrialise America".

The Senators pointed to lingering concerns over the health impact of cannabis, as well as research suggesting that cannabis can be linked to "impaired judgement" and "lack of concentration".

"In light of the documented dangers of marijuana, facilitating the growth of the marijuana industry is at odds with growing our economy and encouraging healthy lifestyles for Americans."

In a separate letter sent to Attorney General Pam Bondi in August, nine Republican representatives argued that "no adequate science or data" exists to support the change.

"Marijuana, while different than heroin, still has the potential for abuse and has no scientifically proven medical value," the letter said. "Therefore, rescheduling marijuana would not only be objectively wrong, but it would also imply to our children that marijuana is safe. That couldn't be further from the truth."

More broadly, polls show that a majority of Americans support efforts to legalise marijuana.

One Gallup poll released in November found that 64% of Americans believe that it should be legalised, although support had drifted slightly from previous years because of a 13-point drop among Republicans.