ScienceDaily (Aug. 18, 2009) — Although exercise is good for your health, extreme exercise may be physically addicting. Rats given a drug that produces withdrawal in heroin addicts went into withdrawal after running excessively in exercise wheels, according to new research. Rats that ran the hardest had the most severe withdrawal symptoms.
http://www.sciencedaily.com/releases/2009/08/090817143600.htm