Addicted to exercise?

Hooked on the runner's high? New research suggests that the body may release chemically "addictive" drugs during exercise. Tufts University psychology professor Robin Kanarek put rats through a training and diet regimen and also injected them with a drug used to treat addiction in humans. From National Geographic:
When injected into human addicts, the drug (naloxone) induces withdrawal symptoms that include writhing, chattering teeth, and swallowing movements...

They found that the most hardcore rat runners showed the greatest degree of withdrawal symptoms, while rats that did not have access to wheels displayed fewer withdrawal symptoms.

Kanarek is not worried about mass exercise addiction in people.

"While we saw naloxone-withdrawal symptoms in active rats, these symptoms were not as severe as those typically seen during morphine withdrawal–suggesting that exercise is not as addictive," she said.

What's more, the addictive effects of exercise could be used in a positive way. "We think a bright side to our findings is that exercise may be one way to actually help [drug] addicts recover," she added.
"Exercise Can Be Like Heroin, 'Gym Rats' Show"