Ketamine, also called "Special K", is a potent animal tranquilizer and a growing thorn in the side of authorities.
"It's been a great increase in the amount that we've seen out in the clubs," says inspector Hilton Burton, the commander of the Washington, D.C., metro police narcotics branch.
Yet police in Washington and other big cities find they lack the legal tools to battle Special K. That's the loophole. The penalty for having small amounts is negligible, so police often confiscate the drug, but let the users go.
"It's a dangerous drug without a doubt, and we try to take as much of it off the street as we can," says Burton. "But in most cases they won't - the U.S. attorney won't prosecute."
So even if users get caught they know they won't get in trouble. That makes Special K in huge demand.
In one seizure at a horse farm veterinary clinic, U.S. drug agents found 7,000 vials - enough to sedate an army of horses or get thousands of kids high.
A ketamine "high" ranges from an out-of-body experience called a "K-hole" to a coma. Because it's an anesthetic, users can hurt themselves and not even feel it. Because it can temporarily paralyze, it's emerged as a date rape drug secretly slipped into a victim's drink.
The problem is so serious, there's a growing market for test strips. Sprinkle a drop of your drink on it, and if it turns bright blue, it means trouble.
"That's what a positive would look like if one of the drugs, specifically in this case ketamine, registered with the card," says Brian Glover, a scientific coordinator for Drink Safe Technologies.
Experts insist Special K is just as dangerous to users as other club drugs that carry stiffer penalties. But for now the laws don't reflect that, making it a sadly convenient drug of choice.