When a person takes pain medication frequently over a long period, the parts of the brain which produce these chemicals naturally diminish and cease to work - the natural painkiller factory has been "outsourced." Sometimes, other factories will try to compensate, such as the adrenal glands. Manic-depression is the result of neurotransmitters being overworked at one point, and exhausted the next; Rarely have you ever heard someone going from depressed mode to manic mode without the introduction of chemicals, such as medication, drugs, or alcohol. ADD however, in my opinion, is the result of an ancestral genetic disposition to react to stimulation at a higher level than considered average. If you were in the African Serengeti, ADD would be very useful, and you would be unlikely to be surprised and subsequently eaten. ADD is the vestage of the hunter, and not necessarily a chemical imbalance, but simply an evolved wiring; a "survival of the fittest" trait. It isn't the inability to focus that defines ADD, (as I see it), but the amazing ability to re-focus very quickly. Unfortunately, while 10,000 years ago, this was paramount to survival, (as proved by your very existence), today, it just annoys the slow-to-react gatherers.