ADDProgrammer is right (as usual). People who have ADD react differently to different meds -- meaning that Ritalin & such don't help everyone with ADD. By the same token, people without ADD react differently to Ritalin & such. So the fact that you feel that you can concentrate better with Ritalin is not an indication that you have ADD.
flintrock also has two points which should be taken into consideration. Ritalin works not only on the brain, but also on the body. If you are having jitters, it means that you are pushing your cardiovascular system beyond where it feels comfortable. Heart problems can follow from too much usage. And addiction is certainly something to keep in mind. Since the drug is prescribed to me for a particular purpose, I am careful to take the minimum amount that will help me calm my chaotic brain without giving me jitters. When I find that I cannot sleep, I cut back on the amount, and I regularly take days off. I don't want to become addicted.
(Just as a side note, here in Germany they're just catching up to the U.S. in prescribing Ritalin for ADD children. For adults, Ritalin is normally prescribed only for narcolepsy. So for the few fortunate adults who can get a doctor to treat adult ADD, we have to sign a form which says that we are taking Ritalin "off-label". So the issue of "off-label" usage of certain drugs has different meanings in different places.

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If the drug is helping you fight your fatigue, then great. And if it's helping you concentrate in school, then also great. Just keep in mind that it's powerful stuff.
--Rheanna