Qnexa: The Next Obesity Drug? How Safe and Effective Is It?

A new obesity drug called Qnexa has won the approval of a panel of physicians appointed by the FDA to evaluate its safety and effectiveness. That puts the drug one step closer to entering the market. Although the FDA doesn’t have to accept the recommendation of the panel, its recent approval increases the likelihood that this new anti-obesity medication will enter the market at some point in the future. If it’s approved, it will be the first new weight loss medication to enter the market in over a decade.

What is Qnexa?

Qnexa is a combination of two medications available only by prescription – phentermine and topiramate. Phentermine is a medication with amphetamine-like effects that is approved by the FDA for treating obesity. It was part of the combination called “fen-phen” that was popular years ago for weight loss. The drug fenfluramine, the fen portion of the combo, was removed from the market after a small number of people using the drug combination developed heart and lung problems. Phentermine was allowed to remain on the market. It works by altering levels of brain neurotransmitters that control appetite and by boosting metabolism.

The other drug, topiramate that makes up Qnexa is commonly prescribed for the treatment of migraine headaches and seizures. No one knows exactly how topiramate promotes weight loss, but it works at least partially by controlling appetite. People who take the medication for migraines or seizures often report that food doesn’t taste as good when they’re on it, and therefore they eat less.

How Well Does Qnexa Work?

One reason Qnexa won such overwhelming support by the panel (they approved it by a margin of 20 to 2) is because people taking the medication lost almost 10% of their body weight after being on the medication for a year. Unfortunately, as with most medications, Qnexa has side effects, some of which could be serious.

Potential Side Effects of Qnexa

Qnexa can cause a rapid heart rate, sleep disturbances and anxiety. This may limit its use in people with heart disease or psychiatric problems. It may also alter the taste of foods and lead to digestive symptoms such as dry mouth, nausea and constipation. Just as concerning is the fact that topiramate is linked with a greater risk for birth defects in women who use it during pregnancy. On the positive side, people taking the medication experience lower blood sugar and triglyceride levels.

Is Qnexa Likely to Be a Safe and Effective Treatment for Obesity?

Preliminarily, Qnexa appears to be one of the more effective weight loss drugs. As with any medication, it’s important to weigh the risks against the benefits. No one knows the long-term risks of using this medication. Only obese people who are at high risk for health problems due to their obesity and who have failed to lose weight after making lifestyle changes should consider using it. It’s not a “quick fix.”

Lifestyle changes including diet and exercise are the safest way to take off extra pounds. One of the dangers of approving this medication is people will see it as a way to lose weight without having to make diet and lifestyle changes. If that’s the case, they’ll likely put the weight back on just as quickly. As with most things, there’s no “free lunch” when it comes to losing weight.


QNexa Advisory Committee Briefing Document.

Physician’s Desk Reference. 2011.

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