NEW YORK — A federal appeals court has ruled that the Drug Enforcement Administration must “act promptly” if formally asked to take another look at laws that consider marijuana as dangerous as heroin or LSD.

The ruling came Thursday in a 2-to-1 vote by judges from the 2nd U.S. Circuit Court of Appeals who agreed that the plaintiffs in a lawsuit against the DEA and other parts of the federal government needed to ask the agency to change its designations for marijuana before bringing the issue to the courts.

The plaintiffs — which include the Cannabis Cultural Association and an Iraq war veteran who suffers from post-traumatic stress disorder — now have an opening to persuade federal authorities to change how they classify marijuana. Many states have legalized recreational pot use, but marijuana is still illegal under federal law.

“It is possible that the current law, though rational once, is now heading towards irrationality; it may even conceivably be that it has gotten there already,” wrote Judge Guido Calabresi.

“A sensible response to our evolving understanding about the effects of marijuana might require creating new policies just as much as changing old ones,” Calabresi added in a majority opinion that included the conclusions of Judge Jed S. Rakoff, a district judge sitting on the Manhattan appeals court temporarily.

Calabresi noted that the plaintiffs claimed that marijuana has extended their lives, cured seizures and made pain manageable.

“If true, these are no small things. Plaintiffs should not be required to live indefinitely with uncertainty about their access to allegedly life-saving medication or live in fear that pursuing such medical treatment may subject them or their loved ones to devastating consequences,” he said.

In urging swift action, Calabresi said plaintiffs had shown that it took an average of nine years to reclassify drugs.

“Although agencies, like legislatures, are often the best decisionmakers, this is so only when they actually do decide,” the majority opinion said.

Calabresi wrote that the panel will retain jurisdiction in the case “exclusively for the purpose of inducing the agency to act promptly.”

In a dissent, Judge Dennis Jacobs mocked the other judges for citing urgency and then giving the plaintiffs six months to ask the DEA to make changes.

He said he viewed as “contrived and fanciful” the claims of the parents of severely ill children who said they suffer fear that they might be subject to federal prosecution because of their involvement in their children’s medical treatment.

“Nobody need fear severe consequences for administering medical marijuana to sick children,” Jacobs said.

“I doubt that the DEA will be hurrying its work on an application that these plaintiffs have not yet filed, seeking administrative action on an old and ramified controversy,” Jacobs said. “As and when this case returns to this Court and this panel, I will be an interested and bemused spectator.”

In a statement, the Cannabis Cultural Association called the ruling “great news.”

“This is the first decision of its kind and will afford the plaintiffs the rapid decision that they and all Americans deserve,” said Nelson Guerrero, the association’s president.