LITTLE ROCK, Ark. – Nearly a third of all cocaine seized in the United States is laced with a dangerous veterinary medicine — a livestock de-worming drug that might enhance cocaine's effects but has been blamed in at least three deaths and scores of serious illnesses.

The medication called levamisole has killed at least three people in the U.S. and Canada and sickened more than 100 others. It can be used in humans to treat colorectal cancer, but it severely weakens the body's immune system, leaving patients vulnerable to fatal infections.

Scientific studies suggest levamisole might give cocaine a more intense high, possibly by increasing levels of dopamine, the brain's "feel-good" neurotransmitters.

Drug Enforcement Administration documents reviewed by The Associated Press indicate that 30 percent of all U.S. cocaine seizures are tainted with the drug. And health officials told the AP that most physicians know virtually nothing about its risks.

"I would think it would be fair to say the vast majority of doctors in the United States have no idea this is going on," said Eric Lavonas, assistant director of the Rocky Mountain Poison and Drug Center in Denver, where as much as half of the cocaine is believed to contain levamisole. "You can't diagnose a disease you've never heard of."

Authorities believe cocaine manufacturers are adding the levamisole in Colombia, before the cocaine is smuggled into the U.S. and Canada to be sold as white powder or crack.

Economic pressures may play a role. Decreased supply in the U.S. has raised cocaine prices and lowered street-level purity. Cocaine traffickers may believe levamisole adds an extra boost to an otherwise weakened product.

Levamisole started showing up frequently in cocaine from Colombia in January 2008. By late last year, the DEA concluded that the spiked cocaine was in wide circulation.

At the same time, hospitals around the country began noticing more cocaine users coming in with agranulocytosis, an illness that suppresses white blood cells necessary to fight off infections.

In Spokane, Wash., a woman in her mid-40s who tested positive for cocaine turned up at a hospital suffering from rashes and other maladies. She eventually died, and the doctor who investigated suspected she had used cocaine laced with levamisole. Doctors suspect levamisole in at least three other illnesses in the Spokane area.

"It's hard to know where this contamination (is), in what part of the country it's located, because there's really no systematic testing for it," said Dr. Joel McCullough, health officer for the Spokane area.

"I don't think it's on the radar of a lot of people, so if there are some other symptoms, I don't know if many clinicians would think to consider that."

Other suspected levamisole deaths occurred in New Mexico and in Alberta, Canada.

Many other people have become gravely ill, including about a dozen patients in Denver and 10 more in Seattle. At least one patient in each city required intensive care or extensive surgery.

In a study published in the Annals of Internal Medicine, researchers suggested that the medication might increase dopamine in the brain, as it has in previous animal experiments.

Levamisole "elevates opiate levels in various brain regions, like codeine and morphine," said Don LeGatt, a clinical toxicologist at the University of Alberta who has studied levamisole in cocaine. "Once you get those elevated, people tend to feel fairly comfortable and not too bad."

The tainted cocaine has received only limited attention in the U.S., though federal authorities are monitoring its use, said Paul Knierim, a DEA spokesman in Washington.

"I think the message is the same: Don't use cocaine, it's a dangerous drug," Knierim said.

Doctors and hospitals remain largely unaware of the cocaine additive. A spokesman for the New Mexico health department said his office plans to write a piece for a Centers for Disease Control and Prevention newsletter in September.

LeGatt said doctors should consider levamisole exposure in cases where otherwise healthy adults or newborn babies come down with infections because of low white blood cell counts. But the levamisole only appears in urine for a few days after exposure, meaning tests should be done as soon as possible.

"When they ask their patients, they're probably going to deny that they use cocaine," he said. "But I think it's important for physicians to persevere to try and elucidate whether or not it is the cause."

In Denver, after word spread about levamisole in cocaine, one man who fell ill asked a doctor if the "animal drug" could have been making him sick. Health officials including Lavonas say the public needs to be warned about the dangers.

"It's not like you can put it on the bottle," he said.