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 US 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 normalize 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, 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-authorized 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.

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 legalized recreational use. But since 1971, cannabis has been a Schedule I narcotic, meaning it has no accepted medical use and a high potential to be abused.

Trump's order drew praise from organizations that have been working to change US cannabis regulations, although the reclassification order has met some resistance from Republican lawmakers who argue it could hinder economic growth due to health concerns.