How do ADHD medications affect those without ADHD?

Since children who have ADHD are supposed to take certain medication in order to help them concentrate as well as improve in school, how would taking these drugs affect the brain of someone who does not have ADHD? For example, many students who are in college or university take ADHD in order to be able to focus better, stay up longer, etc … What would the long-term and short-term effects be of taking these drugs?
Sorry I meant to say that the university and college students illegally take ADHD medications.

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