Most ADD individuals skip doses periodically. Sometimes because they forget (we're treating an illness that causes you to get distracted, remember), or they slept in on the weekend and don't want to mess up their sleep schedule, or they simply don't have anything they really need to be able to pay attention to that day, or maybe they don't need their second dose in the day that particular day.

Unlike, say, antidepressants, there's no real backswing on off days: you just revert to your pre-medication attention levels which, especially in the case of people who got diagnosed later than usual, they have coping mechanisms for.

This results in the average ADD-medicated patient having a couple pills leftover each month.