Nearly a third of all cocaine seized in the United States is laced with a veterinary drug blamed in at least three deaths in the U.S. and Canada.

The deaths have been reported in Washington State, New Mexico and Alberta.

Scientific studies suggest the drug, levamisole, might give cocaine a more intense high.

But it severely weakens the body's immune system, leaving patients vulnerable to fatal infections.

Drug Enforcement Administration documents reviewed by The Associated Press indicate that 30 per cent of all U.S. cocaine seizures are tainted with the drug.

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

Health officials say the vast majority of doctors in the United States have no idea this is going on.

But a spokesman for the DEA says the message is the same, regardless of contamination: cocaine is a dangerous drug.

Since 2008, public health officials in British Columbia, Alberta and Nunavut have also warned of people falling sick after using cocaine contaminated with levamisole, which is used to treat intestinal worms in humans and animals.