Here is a great article from webmd that provides lots of great information on Phentermine.
A little part of the article is printed below but here is the link to take you directly to the article : http://www.webmd.com/drugs/2/drug-4151/phentermine+oral/details
Uses
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. Losing weight and keeping it off can lessen the many health risks that come with obesity, including heart disease, diabetes, high blood pressure, and a shorter life.
It is not known how this medication helps people to lose weight. It may work by decreasing your appetite, increasing the amount of energy used by your body, or by affecting certain parts of the brain. This medication is an appetite suppressant and belongs to a class of drugs called sympathomimetic amines.
How to use phentermine
Take this medication by mouth as directed by your doctor, usually once a day, 1 hour before breakfast or 1 to 2 hours after breakfast. If needed, your doctor may adjust your dose to take a small dose up to 3 times a day. Carefully follow your doctor's instructions. Taking this medication late in the day may cause trouble sleeping(insomnia).
If you are using sustained-release capsules, the dose is usually taken once a day before breakfast or at least 10 to 14 hours before bedtime. Swallow the medication whole. Do not crush or chew sustained-release capsules. Doing so can release all of the drug at once, increasing the risk of side effects.
If you are using the tablets made to dissolve in the mouth, the dose is usually taken once a day in the morning, with or without food. First, dry your hands before handling the tablet. Place your dose on top of the tongue until it dissolves, then swallow it with or without water.
The dosage is based on your medical condition and response to therapy. Your doctor will adjust the dose to find the best dose for you. Use this medication regularly and exactly as prescribed in order to get the most benefit from it. To help you remember, take it at the same time(s) each day.
This medication is usually taken for only a few weeks at a time. It should not be taken with other appetite suppressants (see also Drug Interactions section). The possibility of serious side effects increases with longer use of this medication and use of this drug along with certain other diet drugs.
This medication may cause withdrawal reactions, especially if it has been used regularly for a long time or in high doses. In such cases, withdrawal symptoms (such as depression, severe tiredness) may occur if you suddenly stop using this medication. To prevent withdrawal reactions, your doctor may reduce your dose gradually. Consult your doctor or pharmacist for more details, and report any withdrawal reactions immediately.
Rarely, abnormal drug-seeking behavior (addiction) is possible with this medication. Do not increase your dose, take it more frequently, or use it for a longer time than prescribed. Properly stop the medication when so directed.
This medication may stop working well after you have been taking it for a few weeks. Talk with your doctor if this medication stops working well. Do not increase the dose unless directed by your doctor. Your doctor may direct you to stop taking this medication.