Complex sentence: Long-term misuse of stimulant medications at doses above the therapeutic range for ADHD treatment is associated with addiction and dependence; several studies indicate that untreated ADHD is associated with elevated risk of substance use disorders and conduct disorders.
Split into simpler sentences: Long-term misuse of stimulant medications at doses above the therapeutic range for ADHD treatment is associated with addiction and dependence. Untreated ADHD; however, is also associated with elevated risk of substance use disorders and conduct disorders.