The Department of Health is issuing a warning on the use of synthetic marijuana after people across New York City have been sent to the hospital.

Since last Thursday, officials say 15 people who are believed to have used the substance were admitted to city hospitals. K2, AK-47 or Spice, which are some of the names the drug is known as on the street, became illegal in 2012.

The drug is a chemical produced in a lab that works like marijuana, except it is much more potent, according to Dr. Uri Belkind, of Union Community Health Center. Belkind adds that the labeling on the packaging doesn't typically list ingredients.

This year alone, the Department of Health says it has seen a 220 percent increase in emergency room visits related to the drug. Users can experience agitation, paranoia, numbness and temporarily lose the ability to speak. It can even cause death.

Doctors say the synthetic drug is worse for you than real marijuana. One reason why some believe it is so popular now is because it cannot be detected by a traditional drug test.

The Department of Health is asking residents to report any synthetic marijuana they see on store shelves to police or 311.