joy popping is a slang term for Subcutaneous injection of Heroin or morphine by people who are not addicts. Supposedly there are people who just enjoy an occasional dose of heroin because they want to have a groovy time, or are taking a long weekend off. And supposedly these people can continue this pattern of behavior, using opiates maybe 3 or 4 times a year without ever becoming addicted, even after several years.
These people may be out there. But when news that such a pattern of use theoretically exists, all sorts of kids in their 20's think that they are somehow more mature and psychologically strong then the average dirty junkie, and think they can get away with just using heroin for fun.
And, of course, most of them can not.