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"We can assume from our findings that it is not just cocaine that changes the brain, but that the adulterant levamisole has an additional harmful effect," said research leader Boris Quednow, from the University of Zurich.
"The sorts of cognitive impairment often exhibited by cocaine users may therefore be exacerbated by levamisole," Quednow said in a university news release.
Cocaine is the second-most used illegal substance worldwide after marijuana. Local anesthetic agents, painkillers, caffeine and other substances are often added to street cocaine, the researchers said in background notes.
In Europe and the United States, levamisole is a common additive, possibly because it may increase or prolong cocaine's effects, Quednow and his colleagues suggested.
The researchers analyzed hair samples to determine levels of cocaine and levels of levamisole in study participants. They ended up comparing 26 cocaine users with low levamisole exposure, 49 cocaine users with high levamisole exposure, and 78 people using no drugs.
In tests of mental and thinking skills, regular cocaine users scored worse on attention, working memory, long-term memory, and other mental functions compared to people who didn't use cocaine. But those whose cocaine was cut with levamisole performed worst of all, according to the study.
Moreover, brain scans linked higher levamisole levels with impaired thinking and a thinned prefrontal cortex. This indicates levamisole has a toxic effect on the brain, the researchers concluded.
Although the study didn't prove a cause-and-effect relationship, the authors called on public health officials to expand their drug-checking programs.
"Such programs mean users can have their drugs tested for purity and therefore avoid taking cocaine that has very high levels of levamisole," Quednow said.
The findings were published recently in Translational Psychiatry.
-- Robert Preidt
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SOURCE: University of Zurich, news release, Oct. 31, 2018