The Los Angeles Police Department announced 15 people were hospitalized over the weekend after consuming synthetic marijuana.

Last November, 13 people in San Diego became ill after ingesting synthetic marijuana, also known as synthetic cannabinoids, The Washington Post reports. In April 2015, New York reported 160 hospitalizations related to synthetic marijuana in two weeks. The drug, also known as K2 or Spice, is about 10 times more potent than naturally occurring THC, the psychoactive ingredient in marijuana, the article notes.

From January 1 to March 31 this year, poison centers received reports of 862 exposures to synthetic cannabinoids, according to the American Association of Poison Control Centers. Poison centers received 2,668 calls about exposures to these drugs in 2013, 3,682 exposures in 2014, and 7,794 exposures in 2015.

Synthetic cannabinoids are very different from marijuana, the AAPCC explains. They can cause dangerous health effects, including severe agitation and anxiety; muscle spasms, seizures and tremors; intense hallucinations and psychotic episodes; and suicidal and other harmful thoughts and/or actions.

Synthetic cannabinoids are not tested for safety, and users do not know exactly what chemicals they are using. Chemists who make the drugs are constantly changing the main ingredients, which can change their effects. This makes it especially difficult to treat people who have taken the drugs.