Doctors and multiple prosecutors are on the case, but no one knows who's doing it or why, and whether the victims have been injected with drugs — or indeed any substance at all.

Club owners and police are trying to raise awareness, and a rapper even interrupted his recent show to warn concert-goers about the risk of surprise needle attacks.

It's not just France: Britain's government is studying a spate of "needle spiking" there, and police in Belgium and the Netherlands are investigating scattered cases too.

On May 4, 18-year-old Tomas Laux attended a rap concert in Lille in northern France, where he smoked a bit of marijuana and drank some alcohol during the show. When he came home, he told The Associated Press, he was feeling dizzy and had a headache – and he spotted a strange little skin puncture on his arm and a bruise.