Shares of Vivus Inc. climbed Thursday after the drug developer said pharmacy benefits manager Express Scripts Holding Co. has added its weight loss pill Qsymia as a standard benefit option to its national formulary.

THE SPARK: Vivus, based in Mountain View, Calif., said patients will pay a co-payment of between $50 and $60 for a monthly prescription of Qsymia, which is available only through mail order.

Vivus President Peter Tam said in a statement the Express Scripts decision marked a "major step forward" in expanding access to the drug.

THE BACKGROUND: Express Scripts is the nation's largest pharmacy benefits manager. Pharmacy benefits managers, or PBMs, run prescription drug plans for employers, insurers and other customers. They process mail-order prescriptions and handle bills for prescriptions filed at retail pharmacies.

In September, Qsymia became the second long-term weight loss drug approved in the U.S. since 1999. Competitor Arena Pharmaceuticals received approval for its weight loss pill Belviq in late June. Many doctors have been asking for a new pharmaceutical treatment for weight loss as U.S. obesity rates have climbed.

Shares of Vivus received a boost last month after health insurer Aetna Inc. said it would cover Qsymia and Bleviq as "medically necessary" obesity treatments for patients whose policies don't exclude such drugs.

The Food and Drug Administration initially rejected Qsymia in 2010 over concerns that it can cause birth defects if taken by pregnant women. The agency recommends that women of childbearing age test negative for pregnancy before starting the drug and take a monthly pregnancy test while taking it.

SHARE ACTION: Up 2.4 percent, or 33 cents to $13.97 in midday trading, while the Nasdaq composite index was relatively flat. The company's stock is up 43 percent so far this year.