A synthetic stimulant, such as that present in Phentermine and Phentermine-based diet pills, acts on our Central Nervous System.
It stimulates our nerve cells to release neurotransmitters which are a group of particular chemical compound including adrenalin.
The neurotransmitters signal an acute stress response (also called fight-or-flight response, the two choices a human may have when facing danger) to the body which, in turn, puts a halt to the hunger signal. As a result, it causes a loss in appetite because the brain does not receive the hunger message.