If you’re doing cocaine, chances are this story will not make you quit. But it may make you think twice about your supplier.

A new report by the Centers for Disease control follows 21 cases of the otherwise rare disorder known as agranulocytosis, which is hallmarked by a severe weakening of immune function.

The condition is brought on by the drug levamisole, which used to be given to colon cancer patients. It is also widely used to deworm cattle.

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Citing the Drug Enforcement Agency, the CDC report claimed that 69 percent of all cocaine seized at US borders contains levamisole. The average concentration was near 10 percent. Tainted cocaine was also seized in New Mexico and Washington.

While only one death is known to have been caused by the substance, the report warned that other cases may have gone unnoticed since the DEA first began tracking the increasing rate of levamisole contamination in 2002. Cocaine users, the CDC said, may be less likely to seek medical treatment; and even if they did, the patient may not disclose cocaine use to their doctor.

As for why the drug taints such a large portion of cocaine in the United States, the CDC does not venture a guess. “The reason why levamisole is added to cocaine remains unclear,” the agency said. It added that levamisol was found in less than three percent of heroin seizures, and it was at much lower concentrations than those typically found in cocaine.

“Some studies suggest it intensifies the high by boosting dopamine levels in the brain,” Bloomberg News noted. “Other reports say it is used to dilute, or cut, the drug.”

The CDC said almost all of those suffering from agranulocytosis experienced “fevers and most also had sore throats,” MedPage added. “Nausea, vomiting and diarrhea, and mouth sores were common. A few patients reported pain and/or sores elsewhere in the body.

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“CDC has begun national surveillance for agranulocytosis in association with suspected cocaine or heroin use, collecting information via medical abstraction form and patient interview,” the CDC added. “As of December 15, eight states had agreed to participate.”