Phentermine Side Effects

Side Effects to Report

Some side effects with phentermine, while occurring infrequently, are potentially serious and should be reported immediately to your healthcare provider. These include, but are not limited to:
  • Signs of a life-threatening side effect known as primary pulmonary hypertension, such as:


    • Difficulty breathing
    • Chest pain
    • Fatigue or weakness
    • Dizziness or fainting
    • Ankle or leg swelling
    • Blue lips or skin


  • More outgoing or aggressive behavior than normal
  • Strange behavior
  • Hallucinations (seeing, hearing, or feeling things that are not really there)
  • Agitation or restlessness
  • High blood pressure (hypertension)
  • A rapid heart rate (tachycardia)
  • Feelings of a rapidly or forcefully beating heart (known as heart palpitations)
  • A desire or need to take phentermine more frequently (or at a higher dose) than prescribed.
  • Signs of an allergic reaction, including unexplained rash, hives, itching, unexplained swelling, wheezing, or difficulty breathing or swallowing.

Final Thoughts on Phentermine Side Effects

It is possible that you may experience some or none of the phentermine side effects listed in this article. Unfortunately, it can be difficult to tell whether any particular side effect (such as nausea) is actually caused by phentermine or other factors. Therefore, make sure to let your healthcare provider know if you develop any side effects while taking phentermine or if something "just does not seem right." While it may not be a side effect, your healthcare provider will be able to diagnose and treat the problem.
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