WASHINGTON (AP) — Shares of Vivus Inc. and similar developers of prescription weight loss drugs rose Wednesday after health insurer Aetna said it would cover the medications for certain patients.

Aetna Inc. said in a policy update Tuesday that it would cover Vivus' Qsymia and Arena Pharmaceuticals Inc.'s Bleviq, as "medically necessary" obesity treatments for patients whose policies don't exclude such drugs. Aetna is the third-largest commercial health insurer based on enrollment in the U.S.

Aetna's move could pressure other large insurers to make similar decisions, said Cowen and Co. analyst Simos Simeonidis, potentially expanding the market for the drugs. Still, he said it's unclear exactly how many patients could receive the drugs under the new policy.

With U.S. obesity rates nearing 35 percent of the adult population, many doctors have called for new pharmaceutical treatments for weight loss. In the past year the FDA has demonstrated a new willingness to approve such drugs, though some insurers are still deciding whether to cover the new products.

In September, Vivus' Qsymia pill became the second long-term weight loss drug approved in the U.S. since 1999, and is considered the most effective in a trio of new weight loss drugs. Competitor Arena Pharmaceuticals received approval for its weight loss pill Belviq in late June. A third drugmaker, Orexigen Therapeutics Inc., is still studying another obesity drug named Contrave, which it hopes to submit for approval in 2013.

Shares of Mountain View, Calif.-based Vivus rose $1.31, or 13 percent, to $11.66 in midday trading. Shares of San Diego-based Arena Pharmaceuticals Inc. rose 30 cents, or 3.4 percent, to $9.23. Orexigen shares rose 30 cents, or 6.9 percent, to $4.66.

Other older FDA-approved drugs covered by Aetna's policy include Didrex from Pfizer Inc., Roche's Xenical and GlaxoSmithKline plc's Alli.