WASHINGTON (AP) — Shares of Orexigen Therapeutics Inc. soared Thursday as an analyst started coverage of the drugmaker, attempting to pick winners and losers among the three companies marketing a new generation of weight loss pills.

Vivus Inc. and Arena Pharmaceuticals received federal approval for their anti-obesity pills in June and July, respectively, and they are preparing to launch the products in coming months. Previously the Food and Drug Administration had not approved a new prescription weight loss drug since 1999.

A third California drugmaker, Orexigen, is still running clinical trials of its product, and is working toward an FDA approval date in 2014.

Credit Suisse analyst Lee Kalowski estimates the three drugs and others will garner combined sales of $3.5 billion annually by 2020, according to a research note.

"Yet, even with this substantial growth, many investors are still likely to be disappointed," Kalowski states, pointing out that when $3.5 billion is "split across multiple products, this is insufficient to support even one mega-blockbuster."

Kalowski initiated coverage of San Diego-based Arena with an "Underperform" rating, predicting sales of the company's pill Belviq "will fall short of the market's lofty expectations."

Arena's studies show that Belviq, known generically as lorcaserin, is associated with modest weight loss. On average patients lost just 3 to 3.7 percent of their starting body weight over a year. The analyst also pointed out that the average patient is likely to stay on Belviq for just six months, limiting potential sales. Belviq is scheduled to launch in 2013.

Arena shares fell 41 cents, or 4.5 percent, to $8.75 in trading.

Kalowski initiated coverage of Orexigen with an "Outperform" rating and a $13 price target, saying the stock represents an attractive "risk-reward profile," as it continues study of its drug Contrave.

Sales of La Jolla, Calif.-based Orexigen Therapeutics Inc. rose 49 cents, or 9.9 percent, to $5.40.

Kalowski maintained an "Outperform" rating on Vivus Inc. with a price target of $34. Vivus' Qsymia is generally considered the most effective of the three weight loss drugs, with patients losing 6.7 percent of their body weight in one study and 8.9 percent in another study. The company plans to launch the drug in the coming quarter.

Shares of Mountain View, Calif.-based Vivus rose 77 cents, or 3.5 percent, to $22.94 in trading.