How harmful can it be if i were to take adderall (dextroamphetamine and racemic amphetamine) without a prescription?

Depends: Adderall (dextroamphetamine and racemic amphetamine) at appropriate doses is fairly safe, and brief use at typical doses is relatively low risk. However, it will not make you smarter, though it may help you stay up later to study. Taking if to get high often leads to recurrent use at higher and higher doses, which can be quite dangerous, causing heart problems and psychotic reactions. Better to stick with caffeine.