The common psychostimulants for the treatment of ADHD in children, adolescents and adults (methylphenidate and amphetamines) have no (or extremely rare) toxic effects.
Any medicine could cause allergic reactions or sometimes alterations of the number of red or white blood cells. However, this is extremely rare. The doctors usually make some blood tests at routine visits once or twice a year.
One older psychostimulant (Pemolin) had rare toxic effects on the liver, so a close monitoring of the liver enzyme was necessary.
Methylphenidate (Ritalin, Concerta) does not cause these alterations!