Phentermine is a highly effective diet medication that is the most popular prescription weight loss drug currently prescribed. It functions by impacting the hypothalamus gland, which is located in the lower part of the brain and is connected to the central nervous system. Phentermine functions by interrupting neurotransmitters, which are chemicals that enable communication from one part of the brain to another. By interfering with the neurotransmitters responsible for triggering hunger and appetite, Phentermine reduces the desire to eat and decreases the amount of food necessary for one to feel satiated.

It is believed that the effect of Phentermine to help one feel satiated occurs within the brain by impacting levels of the hormone, leptin. Fat cells secrete leptin in order to assist the body in regulating its weight by communicating appetite, body fat storage and fat burning needs between the body and the brain through the nervous system. It passes the message using the nerves and is transmitted to the hypothalamus gland. The more leptin that is present, the more the brain is signaled that adequate food has been consumed and there is enough fat stored in case of famine or the need for additional energy. Therefore, when leptin is present in adequate quantities, the appetite becomes suppressed and the body will burn some of its fats that have been stored within the adipose (fat storage) tissues.

It is believed that among the many things that phentermine does for the body in order to promote weight loss, raising the leptin levels is one of them. By taking phentermine, people experience the sensation of not being as hungry and being full after eating less food, and that sensation of satiety lasts for a longer period of time.

It is also believed that catecholamine levels are also raised by taking the phentermine, which then stops the transmission of another chemical messenger to the brain called neuropeptide Y. The neuropeptide Y is what encourages the person to feel the need to eat, storing fat from what is consumed, and decreasing energy levels so that less stored fat is burned. By blocking this transmitter, the individual can maintain the full feeling from the previous meal, feeling less hunger and keeping energy levels up, while storing less fat from what is eaten when mealtime comes.