A drug approved to treat cancer and used to de-worm animals is being added to the nation's cocaine supply. Nobody -- at least no public health official -- seems to know why.

The drug, levamisole, can cause a serious weakening of the immune system called agranulocytosis. Twenty-one cases, including one death, were linked to the use of cocaine in four U.S. states between March, 2008 and November, 2009. Here's the paper published this week in the CDC's Morbidity and Mortality Weekly Report. The paper says many more cases likely...