Hollywood’s creeping ketamine habit might have carried on quietly for years, were it not for the death of Matthew Perry. But unlike many celebrities’ high-profile struggles with opiates, cocaine and alcohol, even the most desperate ketamine-dependency cases – and the “Friends” star’s was certainly one – have been consigned to the shadows.

Overdose deaths from ketamine alone are extremely rare, experts say, and no national fatality statistics exist. And yet Perry’s fast ride to deep dependency — an indictment said he shot up six to eight times on the day of his death – was a shock to many.

“That’s a lot of ketamine,” Mike Diamond, an addiction recovery expert who appears on A&E’s…