The Phentermine diet pill stimulates the hypothalamus gland and affects certain neurotransmitters to decrease appetite. Phentermine is one of the most commonly prescribed appetite suppressants out there. As with Phendimetrazine and Diethylpropion, Phentermine should be part of a healthy diet and exercise plan. Talk to your physician if you decide to add one of these supplements to your weight loss plan! Go over side effects to determine which option is best for you.