Drug users in London are being warned that the potentially lethal opioid fentanyl is showing up in other drugs, specifically heroin and marijuana.

Fentanyl is an opioid that's between 50 and 100 times more powerful than morphine. It's now available in powder form similar to cocaine, according to the Middlesex London Health Unit.

"In testing done during the last week of July, we detected fentanyl in the urine of people who told us they weren't even using it. It seems dealers are adding it to other drugs," said Dr. Ken Lee, with the Addiction Services of Thames Valley.

"Users are probably not getting what they think they're getting and someone is going to die from this."

Urine tests conducted this week at ADSTV "detected fentanyl in people who reported using only heroin and in others who reported only smoking marijuana," according to a release from the MLHU.

The health unit, Canadian Mental Health Association, ADSTV and London police are issuing an alert warning people in London of the findings.

"Finding fentanyl in drugs like marijuana means that people who think they are doing something minor may end up dying of overdose," said MLHU Medical Officer of Heath Chris Mackie.

"I don't think anyone is really ready for a wave of overdose deaths in our community," said Mackie. "We've done a good job of getting naloxone out to many partners but there's a lot more work to do."

The MLHU is asking people who use illicit drugs to carry a Naloxone Kit and not use drugs alone.