Two new prescription weight loss drugs, Belviq and Qsymia, now have the FDA's blessing. Which, if either, is for you?

Both drugs help some people lose weight. Neither drug is for everyone. Yet the two drugs are quite different.

Here's WebMD's FAQ comparing Qsymia to Belviq.

How do you pronounce Qsymia? Belviq?

Vivus Pharmaceuticals says you should pronounce Qsymia this way: kyoo-sim-EE-uh. (The company's preferred name was Qnexa, but that was nixed by FDA as sounding too much like other drugs.)

Arena Pharmaceuticals says you should pronounce Belviq this way: BEL-VEEK.

When will Belviq and Qsymia be available?

Vivus says Qsymia should be available "in the fourth quarter of 2012," which begins in September. But don't look for it in your local pharmacy. Because women taking Qsymia must use birth control (see below) the drug will be sold only through "certified pharmacies." These are likely to be major online pharmacies.

Because Belviq has a potential for abuse (see below), the U.S. Drug Enforcement Administration must rule on how to schedule the drug. That process probably began no later than last May. As this usually takes four to six months, Belviq should become available by early 2013. A quicker DEA ruling may speed the drug's arrival.

Why, after not approving any long-term weight-loss drug for 13 years, did the FDA approve two weight loss drugs in one month?

Until relatively recently, most medical researchers considered weight loss drugs to be vanity products. The benefit -- looking better -- was not considered worth very large risks. And early weight loss drugs such as fen-phen carried very large risks, indeed.

Researchers now realize that obesity itself is a disease. Obesity greatly increases all kinds of other serious medical risks, from diabetes to heart disease to depression.

Even though both Qsymia and Belviq carry risks, FDA advisory panels thought long and hard about recommending approval. But the panels were swayed by what most members saw as the much greater risk of untreated obesity.

Do Qsymia and Belviq work the same way?

No. Qsymia and Belviq are very different drugs.

Qsymia combines two currently approved drugs. One is the appetite suppressant phentermine, the safer "phen" part of the infamously unsafe fen-phen diet drug combo.

Phentermine is thought to suppress appetite by triggering release of the brain chemical norepinephrine. This suppresses the appetite by increasing blood concentrations of the appetite-regulating hormone leptin.

The other half of Qsymia is the seizure/migraine drug topiramate. Topiramate causes weight loss in several ways, including increasing feelings of fullness, making foods taste less appealing, and increasing calorie burning.

Belviq causes weight loss by turning on a specific switch that increases levels of the brain messenger serotonin. At dosages intended for weight loss, it does not significantly turn on slightly different serotonin switches responsible for the effects of hallucinogens (such as LSD) and addictive drugs of abuse. Higher doses may trigger these switches, which is why the DEA likely will schedule Belviq as a controlled substance.

Who should and shouldn't take Belviq? Who should and shouldn't take Qsymia?

Belviq and Qsymia are approved for similar problems:

• Obesity, defined as a body mass index (BMI) of 30 or greater.
• Being overweight, with a BMI of 27 or more AND at least one weight-related condition such as type 2 diabetes, high blood pressure, or high cholesterol.

Pregnant women should not take either Belviq or Qsymia.

Qsymia has particular risks for pregnancy, as it can cause birth defects if taken in the first months of pregnancy, even before a woman knows she is pregnant. Women of childbearing age must use effective birth control to keep from becoming pregnant while taking Qsymia.

Qsymia should not be taken by:

• Pregnant women
• People with glaucoma
• People who have been told they have an overactive thyroid
• People taking a type of antidepressant called a MAOI
• People allergic to phentermine or topiramate

Belviq should not be taken by:

• Pregnant or nursing women
• People taking drugs linked to valvular heart disease, such as cabergoline (Dostinex)

Belviq should be taken with caution by:

• People taking certain medicines for depression; migraine; the common cold; or mood, anxiety, psychotic, or thought disorders
• Men with conditions that predispose them to erections lasting more than four hours. These conditions include sickle cell anemia, multiple myeloma, and leukemia
• Men with a deformed penis

Qsymia and Belviq each come with a long list of important safety information, but this list is different for each drug.

Qsymia approval required Vivus to set up a strict Risk Evaluation and Mitigation Strategy (REMS). This program includes a medication guide giving patients important safety information, prescriber training, and pharmacy certification.

Which works better, Belviq or Qsymia?

There's no way to know for sure. Qsymia and Belviq have never been tested in a head-to-head clinical trial.

In the placebo-controlled clinical trials that led to approval:

• People taking Belviq had an average weight loss that was 3% to 3.7% greater than people taking placebo.
• After taking Belviq for one or two years, some 47% of people without diabetes lost at least 5% of their body weight. Only 23% of patients taking an inactive placebo lost this much weight.
• People taking Qsymia for up to one year had an average weight loss of 8.9% over those taking an inactive placebo.
• 70% of people taking Qsymia lost at least 5% of their body weight. Only 20% of patients taking an inactive placebo lost this much weight.

These numbers cannot be used to compare the two drugs, as the clinical trials had different designs.

How long would I have to keep taking Qsymia or Belviq?

People are supposed to keep taking Qsymia or Belviq for the rest of their lives, unless they develop side effects or have other reasons to stop.

If I take Belviq or Qsymia, do I still have to diet and exercise?

Absolutely. In clinical trials, the drugs were effective only when given along with a balanced diet and exercise.

SOURCES: FDA: "Medications Target Long-Term Weight Control."FDA web site, briefing information, Endocrinologic and Metabolic Drugs Advisory Committee.News release, Vivus Pharmaceuticals.News releases, Arena Pharmaceuticals/Eisai Inc.WebMD Health News: "FDA Approves Diet Pill Belviq."WebMD Health News: "FDA Approves Diet Drug Qsymia."WebMD Health News: "New Diet Drug Lorcaserin Wins Vote from FDA Panel."WebMD Health News: "FDA Panel Votes in Favor of Weight Loss Pill Qnexa."

© 2012 WebMD, LLC. All rights reserved.