What is Qnexa&Reg;?

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  • Written By: Melanie Smeltzer
  • Edited By: Daniel Lindley
  • Last Modified Date: 10 May 2019
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Qnexa® is an investigational weight-loss pill that was developed by the California pharmaceutical company Vivus, Inc. This drug is a combination of the prescription medications Topamax® and phentermine, and is said to result in twice the amount of weight loss as other diet pills, as well as combat the effects of related conditions like type 2 diabetes and high cholesterol. Despite this, the drug was rejected for use in the United States in 2010 by the Food and Drug Administration (FDA) as it was found that it might cause several unfortunate side effects.

The two distinct medications, Topamax® and phentermine, that make up the diet pill Qnexa®, have both been said to help users lose weight. Topamax®, or topiramate, was originally created as an anticonvulsant, and was used to treat epilepsy. As time went by it was approved for the prevention of migraines, and a clinical trial suggested it might also be used to help treat post-traumatic stress disorder (PTSD). Phentermine is a short-term appetite suppressant that works by releasing specific chemicals into the brain to reduce the desire to eat. Both medications are said to result in a degree of weight loss; however, it is believed that they work more efficiently together than apart.


A requirement by the FDA states that a drug must produce a 5% increase in weight loss to be considered an anti-obesity drug. During the third portion of clinical trials made by Vivus, Inc., Qnexa® was said to produce an average weight loss of 14.7%. The dosing during this part of the trial was broken down by full, medium, and low strengths. The amount of Topamax® was generally higher, but was given in a controlled-release form, while the phentermine was given in smaller doses but in an immediate-release formulation.

In October 2010, the FDA turned down use of Qnexa® in its current state, as it was thought to produce several harmful side effects. These side effects included memory lapses and thoughts of suicide, as well as heart palpitations and potential birth defects. Despite this, the FDA requested that Vivus, Inc., continue testing and reformulating the drug to address the issues of fetal growth and heart-related complications.

Throughout the testing process Vivus maintained that this weight-loss pill caused very few severe side effects, though the company conceded a number of mild ones. Milder side effects included headaches, stomach upsets, and altered sleep patterns. During clinical trials, the positive effects of the drug were widely thought to outweigh the negative.


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