Phentermine is a stimulant that affects the central nervous system helping to reduce food cravings. This anti-obesity agent is supposed to be used with a healthier diet and regular exercise. If you simply take phentermine, it is not expected to bring any benefits. Your treatment can last for up to 6 weeks. Taking phentermine for longer can result in developing an addiction or not benefitting from the dose as much as before. Make sure you tell your doctor if you have a history of drug abuse, heart disease, hyperthyroidism, glaucoma, diabetes, arteriosclerosis, high blood pressure, or any other medical conditions that may affect the dose you are prescribed. Also tell your health care provider about any other prescription or over-the-counter medications you are using or intend to use, including but not limited to: MAO inhibitors, medications for weight loss and depression, insulin, guanethidine, fluoxetine, paroxetine, fluvoxamine, vitamins, and sertraline. Some of the drugs listed can be combined with phentermine at lower doses, while other ones can cause a potentially lethal reaction. MAO inhibitors (tranylcypromine, isocarboxazid, selegiline, phenelzine, and rasagiline) must never be combined with phentermine, as it can be very dangerous. If you are not sure whether the medicine you are taking is a MAO inhibitor, talk to your health care provider or your pharmacist to be sure your treatment is safe. At least 2 weeks should pass between the last dose of a MAO inhibitor and phentermine. Taking more of this medicine is not going to make your treatment more effective. Make sure you always take the dose recommended and avoid doubling up your dose even if you missed the previous one. If you have been using this medicine for several weeks and need to stop the treatment, make sure you do so gradually. Gradual withdrawal is recommended to avoid such symptoms as extreme tiredness and depression. Sometimes side effects are possible. Mild side effects of Phentermine are likely at the beginning of the treatment and can include the following ones: unpleasant taste, diarrhea, vomiting, dry mouth, and constipation. Make sure you throw away any phentermine you did not use, as otherwise taking it can produce dangerous health effects. This medicine is FDA pregnancy category C. It is not known whether phentermine can harm to an unborn baby, but it does pass to breast milk and may be dangerous for a nursing infant.