A statewide ad blitz by the group pushing for legalized marijuana warns Floridians of the dangers of black-market pot: It may be laced with fentanyl, they say, a potent drug driving an epidemic of fatal drug overdoses. “I’ve seen it firsthand,” Gadsden County Sheriff Morris Young said in a 30-second television spot running in primetime. Legal pot will be regulated and lab-tested, he said, making his case for why voters should support Amendment 3 in November to authorize recreational marijuana in Florida. But amid heightened concerns across the country about the synthetic opioid fentanyl, usually taken separately or mixed with heroin or other opioids, experts unaligned with the campaign cast serious doubt on the claim that fentanyl-tainted marijuana is an actual problem. Several officials on the frontlines of the opioid epidemic say they haven’t seen deaths caused by fentanyl-laced marijuana.