Stimulants such as Ritalin and Adderall are prescribed to those diagnosed with ADD/ADHD because the part of their brain that controls attention is not working properly, to put it in real basic terms. It sounds contradictory to use a stimulant for someone who may be hyper or lack attentiveness but in reality the drugs stimulate neurotransmitters to work properly with dopamine.

Now, our brains are very complex and inattentiveness can be due to plenty of other factors including depression and anxiety or behavioral and enviornmental factors or a combination of all of the above.

Someone without basic ADD/ADHD (regular people) will not respond properly to these stimulants and can infact have averse results. Too high or too low levels of dopamine will have negative consequences.

Here is a quick video explaining how stimulants affect the brain: https://youtu.be/MeJRBsghMt8#t=1m22s

Now, with depression, stimulants are not commonly prescribed but anti-depressants. Anti-depressants operate on the neurotransmitter Serotonin. (Selective serotonin reuptake inhibitors). These stimulate in other ways by decreasing inhibition. Basically, being depressed can cause one to have a lack of initiative and follow through. This can often appear like ADD but is actually different. This is why it's very important to work with a therapist and psychiatrist to tuss out these symptoms and causes. Also, no pill is a magic fix all and has to be combined with behavioral changes.

Here is a very basic short video about anti-depressants/SSRIs and how they work https://youtu.be/G4r3qCkLUDQ.

The funny thing about exercise, is that it can be a positive feedback loop but is hard to get started. It is also a very good mitigator and reliever of depression, even if temporary. The physical effects of doing cardiovascular activity have multiple benefits and that includes mentally/emotionally.