Does phentermine work as well as claimed?

Someone looking to lose weight may ask, “does phentermine work?”

First, a brief note on what it is. It is marketed as a weight loss drug and an appetite suppressant. Drug companies market drugs all the time though, so does phentermine work as advertised?

Weight Loss Through Appetite Suppression

  • Does phentermine work as an appetite suppressant?
    In fact, yes. It both reduces the desire to eat, and makes users feel full sooner. Clinical medical research has shown since 1959 that it is one of the most effective appetite reduction drugs available.


  • How does phentermine work?
    It makes users feel less hungry, so they eat fewer calories. The body gains weight when calories taken in are greater than calories that get burned. Weight is lost when we burn more calories than we eat. It helps users control their urge to eat, so they end up eating fewer calories. It also releases two types of adrenaline. Adrenaline causes the body to release stored fat energy. This means that the body is burning fat rather than adding more fat. Fewer calories taken in and more calories burned means users of this diet drug begin losing weight.
  • Does phentermine work by itself?
    This drug does help users lose weight by itself. It is more effective when added to other drugs such as topiramate. Regular exercise and a healthy diet also make phentermine’s effect stronger. Doctors will often suggest several of these approaches.

Safety, Adverse Effects, & Drug Interactions

  • Does phentermine work safely?
    The risk of side effects for this drug is quite low. Doctors are sure enough of its safety to continue offering it to users. However, there are risks to taking diet pills in general. Most of these risks come in the form of stress or damage to the heart. The United Kingdom banned some diet pills when this was discovered. The United States has banned several other drugs from being combined with phentermine as well. Tests show that pregnant women should not ever take phentermine. It is usually safe if these risks are watched for.
  • Does phentermine work with other medicines?
    Medications that can stress the cardiac system shouldn’t be combined with phentermine. MAOI antidepressants in particular should not be used with some diet drugs. They may also inhibit or stop the effectiveness of drugs such as clonidine, methyldopa, and guanethidine. Again, the patient should speak to their doctor about phentermine’s effects and fully disclose what medications they are taking by prescription and over the counter.

woman measuring waist

Worth It, But Be Aware

Again, healthy diet and exercise are the best ways to lose weight. Using a diet pill does help patients lose weight faster. Users eat less food and burn calories. If a user is not losing weight with exercise diet pills can help.

It does come with some risks though. Heart conditions can be made much worse by this weight loss drug. Fast and uneven heart rate is one result. In rare cases users can have an infarction. Chest pains are also a potential problem. The heart valves can be damaged with long time use. Because of this most doctors will only prescribe it for short terms of 12 weeks. Because it also causes problems with other drugs, patients should tell their doctor everything they are taking.

Does it work? Yes, it does. It works best when added to other healthy life choices. It has very serious risks, but these side effects are also rare. If someone has had trouble losing weight, they should talk to their doctor about the drug.