(Reuters) - Both advocates and opponents of legalized marijuana reacted with caution on Saturday to signs from the White House that growers in U.S. states where the drug is permitted would be shielded from federal prosecution, saying it was too early to know the final impact.

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U.S. Senator Cory Gardner of Colorado announced on Friday that he had convinced U.S. President Donald Trump, a fellow Republican, to protect from federal interference those state laws that legalize marijuana for certain uses.

Last year, U.S. Attorney General Jeff Sessions, who opposes marijuana use, rescinded a memo issued by Trump’s Democratic predecessor, Barack Obama, that dialed back enforcement of the federal ban in states that legalized the drug. That decision unnerved the fast-growing U.S. marijuana industry, which has been legalized in more than half of all states.

“Trump’s pledge to Gardner is a significant and potentially game-changing development but it does not necessarily mean that Sessions it no longer a threat to licensed cannabis businesses,” Mike Liszewski, a policy advisor at the pro-marijuana Drug Policy Alliance, said in a statement. He said, however, that the agreement made it “even more politically difficult for Sessions to initiate a crackdown.”

He joined advocates on both sides saying it remained to be seen what final legislation would look like. Gardner said on Friday Trump had pledged to sign into law a bill that would codify states’ rights to set their own marijuana legislation.

Tom Angell, a founder of the Marijuana Majority, said he was equally annoyed by those saying the announcement was a fait accompli and those who dismissed it as empty.

“The correct posture is: This is hugely positive, but it’s going to take focused, hard work and pressure to make sure words become reality,” Angell wrote on Twitter. “Let’s make him do it.”

Opponents of legalized marijuana have said federal oversight will be necessary for at least as long as black markets exist. They also note that some of the legal production ends up crossing state lines anyway.

Jeff Hunt, director of the Colorado conservative think tank the Centennial Institute, said Colorado law enforcement does not have the resources to deal with marijuana regulation.

“The president is supporting the rights of Coloradans to make decisions for themselves,” Hunt said in a telephone interview. Hunt, who critically compares the burgeoning marijuana industry to the tobacco industry, said he believed Trump still backs federal prosecution for “egregious violations of federal law when it comes to recreational marijuana.”