Lay off that whiskey, and let that cocaine be.

That’s how the 1927 song made famous by Johnny Cash goes, but maybe add the line, if you’re going to do cocaine, try not to do it with animal drugs.

KTLA in Los Angeles reports doctors are warning cocaine users about product cut with levamisole, used for deworming livestock.

The warning follows reports of several patients developing serious skin reactions after smoking or snorting cocaine believed to be contaminated with the veterinary drug.

The report, published online in the Journal of the American Academy of Dermatology, said six patients developed purple-colored patches of necrotic skin on their ears, nose, cheeks and other parts of their body and, in some instances, suffered permanent scarring after they had used cocaine.

According to the U.S. Department of Justice, up to 70% of cocaine in the U.S. is contaminated with levamisole, which is cheap and widely available.