Three pharmaceutical companies are vying to get the U.S. Food and Drug Administration's approval for their anti-obesity pills.

Vivus Inc.'s pill Qnexa will be subjected to a public review on Thursday while Arena Pharmaceuticals Inc.'s lorcaserin is scheduled for an FDA panel review in September. A third diet pill, Orexigen Therapeutics Inc.'s Contrave, will have its FDA panel review in December.

The three drugs have to hurdle FDA's main criteria for approval. They should  reduce total body weight by at least five percent after one year without risky side effects.

So far, Qnexa is leading the race as clinical trials showed it can reduce body weight from 13 to 15 percent.  The amphetamine and an anticonvulsant combination, however, have memory and concentration problems as side effects that led to patient dropping out of the trials.

The antidepressant and an anticonvulsant drug Contrave produces between 5 and 10 percent weight loss but has nausea as main side effects.

Lorcaserin, a brand new drug that acts on serotonin or the brain chemical associated with feelings of well-being and satiation, produces the lowest weight loss at only five percent of body weight and had the least side effects.

There h as been no new diet pill for 13 years since fen-phen, which was pulled off the market in 1997 due to links to heart valve damage and lung problems.