When taken as directed by a medical professional, and with the necessary lifestyle changes Phentermine will work more effectively. Phentermine can be an effective weight loss supplement when combined with a healthy diet and exercise routine. As your body sheds water weight, you can lose several pounds in the first week if you change your lifestyle. One to two pounds should be lost per week. Slow and steady weight loss ensures that your body loses fat, not muscle. Taking Phentermine in the morning as prescribed should suppress appetite throughout the day.