Studies have shown that you can be addicted to exercise
This is actually true, I know someone that was completely addicted to exercise, to the point they could not go one day without execising. They cried, whenever they coudln't go to the gym, or were ill and had to stay in bed, rather than being able to exercise.
Addiction, and dependance are very similar to each other, and both have a psychologcal element, in that, it can make you feel better, about yourself, about life, or whatever it may do, and this is usually what gets someone to continuosly go back to their habits. When it's to do with mind-altering drugs, they can of course, create chemical inbalances, which can make things even worse, and make it much more difficult to get rid of the habit, the dependance. Drugs like heroin, have painful withdrawal symptoms, and this makes getting off the drug extremely difficult, along wih having to face all of the psychological problems that are connected with it.
One more thing, is that the person may have become hooked on drugs, because of trouble in the rest of their life, and this can also be the case with any other dependance, or addictgion, including computer/internet addiction.