Phentermine and topiramate extended-release (long-acting) capsules are used to help adults who are obese or who are overweight and have weight-related medical problems to lose weight and to keep from gaining back that weight. Phentermine and topiramate extended-release capsules must be used along with a reduced calorie diet and exercise plan. Phentermine is in a class of medications called anorectics. It works by decreasing appetite