I have been using drugs when I was young as well as being a distance runner and, well, runner's high is very similar to artificial intoxication. In fact, nothing I've ever tried has given me the same level of intoxication and euphoria as a run over 70 minutes long. There are studies that show similarities between a runner's high and opioid intoxication but I never tried opioids so I can't tell anything about it from experience.

The problem was that I quickly became addicted to running, I would feel extreme depression if I didn't run in the morning and I would often go for a second run as I was craving the pleasure. During my two years of distance running I fantasized about becoming a social outcast who does nothing but runs. Which is what I was, actually. I didn't have many activities aside from running.. I was eventually kicked out of the job I had because I had to go for a team workout in the middle of the day, which screwed up my job performance. Friday nights, Saturday nights, and Sunday nights were spent pounding the pavement.

And I wasn't skinny either, 40-50mpw with workouts sky-rocketed my appetite so I would go on a "Runner's diet": eating everything that was edible. I have lost a lot of weight since I stopped running :)

This might be a controversial view but I do think that running can lead to addiction and even though it's more socially acceptable (not to mention cheaper) than, say, heroin, like any addiction it has its downsides.