Phentermine belongs to the anorectics, or appetite suppressants, class of drugs. Taking phentermine suppresses your appetite, allowing you to eat fewer calories. This can result in weight loss over time. While the precise mechanism by which phentermine reduces appetite is unknown, the drug is thought to work by increasing neurotransmitter levels in your brain. Neurotransmitters, which include the chemicals norepinephrine, serotonin, and dopamine, are the chemical messengers in your body. When your levels of these three chemicals rise, your desire to eat decreases.