Phentermine is used by certain overweight people, such as those who are obese or have weight-related medical problems. Losing weight and keeping it off can lessen the many health risks that come with obesity, including heart disease, diabetes, high blood pressure, and a shorter life. Phentermine is not known how this medication helps people to lose weight. Phentermine may work by decreasing your appetite, increasing the amount of energy used by your body, or by affecting certain parts of the brain. This medication is an appetite suppressant.