Phentermine is a prescription medicine used to lessen appetite.it can help weight loss. The average weight loss when taking phentermine is 3% of initial body weight after 3 months and 5-7 after 6 months. Phentermine working in the body within 3 to 4 hours. Adults and children 17 years of age and older 15 to 30 milligram once a day. Adipex-p and generic tablet dosage is 37.5 mg by mouth once daily. This drug may rearly cause serious lung and heart problems.