So far, use of krokodil seems largely confined to Russia and other former Soviet republics. Besides an apparent case in Massachusetts in April, addicts in the United States have not taken to krokodil so far.
Scientists know very little about the homemade drug. Its psychoactive agent, desomorphine, was first synthesized in the United States in 1932 in the hope of finding a substitute for morphine that would be less nauseous and less addictive.
But desomorphine is eight to 10 times as potent as morphine, and its effects come and go more quickly, which may be why the new drug proved even more addictive than the one it was intended to replace.
The body metabolizes desomorphine quickly, which makes it difficult for doctors to know for sure whether someone has used krokodil.
While the recent cases in Arizona are officially unconfirmed, the patients told physicians that they had taken the drug, according to a doctor at the Banner Poison Control and Drug Information Center in Phoenix.
Desomorphine, however, is not why krokodil is so dangerous.
About 10 years ago, Russians apparently discovered how to synthesize desomorphine at home using commercially available ingredients including red phosphorus, which they reportedly glean from the sides of matchboxes, and codeine, which until last year was available over the counter in Russia. The resulting substance contains several caustic byproducts.
"When you use the krokodil . . . really what you're doing is injecting red phosphorus and solvents into your body," said Matt Zuckerman, a toxicologist at the University of Massachusetts Medical School.
With regular use, those toxins can rot flesh, causing abscesses and gangrene.
A recent U.N. report attributed krokodil's emergence to a heroin shortage in Russia. The country's head drug official has said that a ban on over-the-counter sales of codeine has reduced krokodil use.
Zuckerman suggested that krokodil is unlikely to become popular in the United States. "Tragically, we have an abundance of heroin, and an abundance of OxyContin, oxycodone, and other opiates," he said, adding that these drugs pose a much greater overall risk to public health than krokodil.