Glad I don't live in New York. Not sure about the details here - MSNBC deployed their usual crack staff of reporters to bring you all the details - but apparently it's illegal for a dentist to get smashed on his own stash. I can only assume it's legal to have and administer nitrous to patients in NY, but illegal to take it yourself if you're the doctor prescribing it.
Of course, nitrous is really REALLY harmless, has effects that last less time than the afterglow of an orgasm, and is freely available to any dental patient with tartar and a bit of panic who need to drive home after their appointments - I'm not sure why the good doctor was arrested. Like I said, glad I don't live in NY.