Phentermine (phenyl-tertiary-butylamine), with several brand names including Ionamin and Sentis, is a medication used together with diet and exercise to treat obesity. It is taken by mouth for up to a few weeks at a time, after which the benefits subside. It is also available as the combination phentermine/topiramate. Common side effects include a fast heart beat, high blood pressure, trouble sleeping, dizziness, and restlessness. Serious side effects may include abuse, but do not include pulmonary hypertension or valvular heart disease, as the latter were caused by the fenfluramine component of the fen-phen drug combination. Use is not recommended during pregnancy or breastfeeding, or with SSRIs or MAO inhibitors. It works mainly as an appetite suppressant, likely as a result of being a CNS stimulant. Chemically, phentermine is a substituted amphetamine. Phentermine was approved for medical use in the United States in 1959. It is available as a generic medication. In 2020, it was the 181st most commonly prescribed medication in the United States, with more than 3million prescriptions. Phentermine was withdrawn from the market in the United Kingdom in 2000, while the combination medication fen-phen, of which it was a part, was withdrawn from the market in 1997 due to side effects of fenfluramine which caused increased levels of circulating of serotonin which stimulated serotonin receptors on heart valves and thus causing valve insufficiency and leading to primary pulmonary hypertension (PPH). According to the NIH (National Institutes of Health) there is no evidence that phentermine causes PPH. Medical uses Phentermine is used for a short period of time to promote weight loss, if exercise and calorie reduction are not sufficient, and in addition to exercise and calorie reduction. Phentermine is approved for up to 12 weeks of use and most weight loss occurs in the first weeks. However, significant loss continues through the sixth month and has been shown to continue at a slower rate through the ninth month. Contraindications Phentermine is contraindicated for users who: have a history of drug abuse are allergic to sympathomimetic amine drugs are taking a monoamine oxidase inhibitor (MAOI) or have taken one within the last 14 days have cardiovascular disease, hyperthyroidism, or glaucoma are pregnant, planning to become pregnant, or breast-feeding. Adverse effects Tolerance usually occurs; however, risks of dependence and addiction are considered negligible. People taking phentermine may be impaired when driving or operating machinery. Consumption of alcohol with phentermine may