The powerful opioid fentanyl is far stronger than heroin — about 50 to 100 times more potent than morphine.

The deadly drug is a legal painkiller that's approved to treat severe pain, usually in cases of advanced cancer. But in recent years, it has been increasingly found mixed with heroin and other opioid painkillers, making the drugs far stronger and riskier.

Fentanyl is also now being cut into cocaine, and is causing a rise in overdose deaths of occasional users who likely have no idea that they've ingested the painkiller in the first place, according to the New York City Department of Health (NYC DOH).

Since fentanyl is easy and inexpensive to make, drug dealers are using it to spike other substances and create counterfeit versions of drugs.

Dr. Samuel Gutman, who works with a group that provides medical services at large events, told Vice Canada that dealers have started to cut cocaine with fentanyl since "it's cheap and available, and it's easy to synthesize."

In 2016, 44% of the more than 1,300 overdose deaths in New York City involved fentanyl. And 37% of total overdose deaths involved cocaine and fentanyl without heroin — up from 11% the year before.

"All New Yorkers who use drugs, even if only occasionally, should know their drugs may be mixed with fentanyl," health commissioner Dr. Mary Bassett said in a statement to Business Insider.

Coroners and health officials in Cincinnati, Rhode Island, and Vancouver have also all issued warnings about the recent surge in fentanyl-laced cocaine. The trend is especially concerning given that non-regular opioid users are often more sensitive to a small amount of fentanyl.

Just a few milligrams of fentanyl is enough to vastly increase the overdose risk for any user. Opioid painkillers can slow and stop breathing, which is why they carry a high risk compared to other drugs — and fentanyl is a particularly potent opioid.

CDC officials say that over the past several years, painkillers laced with fentanyl have led to an unprecedented rise in opioid-related deaths. After musician Prince's overdose, investigators found pain pills that tested positive for fentanyl even though they were marked as hydrocodone (another powerful but less deadly opioid) and acetaminophen (found in Tylenol).

As part of its effort to combat the overdose epidemic, New York's health department has launched a mobile app that directs users to programs and pharmacies where they can get naloxone — the antidote that can reverse an opioid overdose — without a prescription.

"The destruction wrought by substance use disorder in our state is horrific; it's reaching pandemic levels with overdose deaths outpacing traffic deaths," said Assembly member Linda B. Rosenthal in a statement emailed to Business Insider. "Fentanyl-laced cocaine is the latest front in this battle, and increased public consciousness of this new and growing threat is imperative."