Phentermine is a weight loss medication that works by reducing appetite and increasing energy expenditure. It is generally prescribed for people who are obese and have not been able to lose weight through diet and exercise alone. Although it can be effective, phentermine is not without side effects, including dry mouth, constipation, insomnia, and increased heart rate. It can also be habit-forming and potentially addictive, which is why it is usually only prescribed for short-term use. In order to get the best results from taking phentermine, it should be used in conjunction with a healthy diet and regular exercise. As with any medication, it is important to discuss possible risks and benefits with your doctor before starting treatment with phentermine.