Phentermine is a stimulant that is chemically like amphetamine and carries a risk of dependence and abuse, which is why it is a controlled substance and only available on prescription. But the potential for dependence and abuse is lower with phentermine than other stimulants, such as amphetamine, methylphenidate, or narcotics, which is why it is a Schedule 4 controlled substance, rather than being in a higher schedule class, such as schedule 2 or 3.