Toronto police are warning the public that heroin being sold on the streets of downtown Toronto may be laced with fentanyl.

The warning comes after a 28-year-old man died of an overdose on Wednesday near Queen and Bathurst streets.

Police say the man consumed a “substance suspected to be heroin” that may have been laced with fentanyl.

“Anyone coming into contact with a substance suspected to be heroin should exercise extreme caution,” police said in a release.

Fentanyl can’t be seen, smelled or tasted, making it difficult to detect when mixed with other drugs. It’s 50 times stronger than heroin.

On Wednesday, police in Ottawa issued a warning after counterfeit pills manufactured to look almost identical to prescription opioids like Percocet were found to contain fentanyl.

Here are some more facts about fentanyl.

What is fentanyl?

Fentanyl is a powerful synthetic opioid traditionally used for pain management. Fentanyl can’t be seen, smelled or tasted, which makes it difficult to detect when laced into other drugs. Fentanyl functions by binding to opioid receptors in the brain to boost levels of the neurotransmitter dopamine, which produces feelings of euphoria and relaxation.

Where does it come from?

Street fentanyl comes from one of two sources: pharma-diverted or illicit. The first category refers to pharmaceutically produced fentanyl, whether patches or injection grade, that is diverted from the medical stream to be sold on the black market. Illicit fentanyl refers to the drug that is synthesized. It is believed a high proportion of the fentanyl in Canada originates from China.

Why is it so dangerous?

Besides producing a drug high, opioids such as fentanyl also depress the body’s rate of respiration, which can cause breathing to stop. Fentanyl’s potency can be amplified when mixed with other substances, such as cocaine or heroin, which makes the consumption of drugs that are unknowingly laced with fentanyl especially dangerous. Miniscule variations, between a grain of fentanyl and two grains, can make the difference between life and death.

What is being done to address the overdose crisis?

Some provinces have been distributing naloxone kits throughout their communities. Naloxone is an opioid antagonist that reverses the effect of an opioid, which when used abruptly ends any drug high but also restores respiration to normal levels. Some jurisdictions have also made applications to open supervised-consumption sites, where drug users can use substances under supervision.

How many people have died from fentanyl overdoses in Canada?

There are no conclusive, up-to-date statistics on opioid overdoses, both fatal and otherwise, in Canada. In British Columbia, there were 559 deaths from illicit-drug overdoses between January and September, 332 of which were linked in whole or in part to fentanyl. That’s nearly three times the number of fentanyl-related deaths that occurred over the same period last year.

Sources: Government of Canada, Know Your Source, National Institute on Drug Abuse, B.C. Coroners Service