Synthetic cannabinoids are man-made, mind-altering chemicals that some users turn to as an alternative to marijuana. They are either sprayed on dried, shredded plant material so they can be smoked or sold as liquids to be vaporized and inhaled in e-cigarettes and other devices, according to the National Institute on Drug Abuse.

The little packets of "fake weed" — with harmless names like K2, Spice, Mr. Happy, Scooby Snax, Kush, and Kronic — can be purchased online as incense or natural herbal products. The unregulated substances are intended to produce the same effects as illegal drugs like marijuana.

These chemicals are called cannabinoids because they act on the same brain cell receptors as tetrahydrocannabinol (THC), the main ingredient in marijuana. But, synthetic cannabinoid chemicals differ from THC and may affect the brain in unpredictable ways and can be toxic.

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Last week, more than 160 overdoses with 10 deaths happened in Philadelphia. The heroin used may have been contaminated with a mix of fentanyl and a toxic synthetic cannabinoid,  5-fluor-ADB.

The Centers for Disease Control and Prevention warns that anyone who has purchased a synthetic cannabinoid product since March 2018 should not use it.

A number of states have reported multiple cases of severe bleeding among people who have used contaminated synthetic cannabinoids, including some deaths, according to the CDC.

Breathing and gastrointestinal problems, heart attacks, stroke, kidney failure, and muscle damage have all been attributed to the use of synthetic cannabinoids. They can also affect brain function and leave the user agitated, irritable, or confused. Users may suffer hallucinations, have suicidal thoughts, delusions, or become violent.

There are no standards for making, packaging, or selling the drugs, so two similar packets may contain different chemicals. Synthetic cannabinoids may also be contaminated with other drugs or toxic chemicals, such as "bath salts," and "flakka," synthetic cathinones.

The long-term effects of synthetic cannabinoid use are not known.