The usual adult dose is 15 mg to 30 mg at approximately 2 hours after breakfast for appetite control. Late evening medication should be avoided because of the possibility of resulting insomnia. Description Phentermine is used for weight reduction in patients with obesity. It is used in patients with obesity who have not been able to lose weight with diet and exercise alone. This medicine works by suppressing your appetite. This medicine is available only with your doctor's prescription. Phentermine requires a prescription from a licensed healthcare professional to purchase at your local pharmacy. This usually means that you need an in-person consultation with a medical professional who can write you a prescription. Learn about phentermine (Adipex-P) usage and dosing. Read the latest news and reviews about the drug as well as potential side effects and popular alternatives. Phentermine is used with a doctor-approved exercise, behavior change, and reduced-calorie diet program to help you lose weight. It is used by certain overweight people, such as those who are obese or have weight-related medical problems. Phentermine is an appetite suppressant used together with diet and exercise to treat obesity. Learn about side effects, interactions and indications. Overview: Phentermine is used short-term to help with weight loss in people who have obesity or are overweight. Common side effects include fast heartbeat, high blood pressure, and restlessness..