Marijuana is legal but police are warning there is a chance it could be laced with fentanyl.

This is the first time fentanyl has been found in marijuana in Connecticut, but police say it won't be the last.

Marijuana rarely causes an overdose, but it's happened to dozens of people across the state since July.

"We saw the overdoses with people who were saying they were just smoking weed and overdosed," said Brian Foley, of Connecticut Department of Emergency Services and Public Protection.

One of those marijuana samples tested positive for fentanyl - a deadly drug that typically had only been spotted in heroin and cocaine.

"It slows down your breathing. You can eventually die because your breathing becomes so slow that you die basically on respiratory arrest," said Susan Logan, Department of Public Health epidemiologist.

Now the state crime lab will test all marijuana samples for fentanyl.

But what's behind this trend? Police say street dealers are getting more aggressive now that marijuana is legal.

"To compete with the over-the-counter sales, with the medical marijuana industry, the dispensaries," said Foley. "They are not worried about their sales base. There are plenty of people waiting in line to buy."

That's especially dangerous for teens who only have access to the black market.

Health officials say they have Narcan ready in case of an overdose.

If you're a parent, knowing the warning signs of an overdose is critical.

"If you look at their eyes -- pinpoint pupils. They might have blue lips and fingertips," said Logan.