Two new studies suggest synthetic marijuana, also known as “K2” or “Spice,” may cause kidney damage. The studies were presented at a meeting of the National Kidney Foundation, HealthDay reports.

“Use and abuse of these products have been tied to acute kidney injury in patients across the country,” Kerry Willis, PhD, Chief Scientific Officer of the National Kidney Foundation, said in a news release. “Despite being legal and marketed as safe, it appears these products are far from it.”

Synthetic marijuana is usually comprised of herbal plant material that has been sprayed with chemicals that mimic THC, the main psychoactive substance found in natural marijuana.

“Currently we do not know how these agents, either the herbs used or the chemicals sprayed, affect the different tissues of the body as scientific studies are lacking,” said one of the studies’ authors, Manuel Fernandez Palmer, MD, of Methodist Dallas Health Center. “Theories suggest that the compounds may have harmful heavy metal residues as these are known to affect different parts of the body, including the kidneys.”

The studies describe patients who used synthetic marijuana and quickly developed acute kidney injury requiring emergency dialysis. “While there is no definitive proof that synthetic cannabinoids were the cause of the kidney injury, these observational studies strongly support that there is a correlation between the two,” Dr. Fernandez said.

In 2013, the Centers for Disease Control and Prevention said kidney damage caused by synthetic marijuana was reported in 16 patients in six states. 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.