Kidney damage caused by synthetic marijuana was reported in 16 patients in six states last year, according to the Centers for Disease Control and Prevention (CDC). All were admitted to the hospital, and five required hemodialysis, a treatment for advanced kidney failure. Hemodialysis involves filtering a person’s blood to remove waste and extra fluids, and returning the clean blood to the body.

All but one of the patients were male. They ranged in age from 15 to 33, and none had a history of kidney disease, HealthDay reports.

According to the CDC report, synthetic marijuana has become increasingly popular because it is thought to produce a more intense high than marijuana, is easily accessible and affordable, and isn’t picked up by many commonly used urine drug tests.

Earlier this month, researchers from the University of Alabama at Birmingham reported four cases of previously healthy young men, whose acute kidney injury was associated with synthetic marijuana, such as Spice and K2. The men came to the hospital with symptoms of nausea, vomiting and abdominal pain after using synthetic marijuana. All four men recovered kidney function, and none required dialysis.