WASHINGTON - Shares of Amgen Inc. were up almost 9 percent early this afternoon, a day after the Food and Drug Administration approved Amgen's bone strengthening drug, Prolia, for postmenopausal women at risk for fractures, a potential blockbuster market.

The injectable drug is given once every six months to increase bone mass and strength.

Shares of the Thousand Oaks, Calif.-based company were trading on the Nasdaq market at $55.20, up $4.44 (8.75  percent), with 12 million shares trading hands - almost double the daily average for the last three months. The stock has traded in the $53.31-$55.37 range so far today, and has been in the $49.21-64.76 range over the last 52 weeks.

Analysts think Prolia should bring Amgen billions in revenue after it is launched. Sales of the company's former top sellers, the anemia drugs Aranesp and Epogen, have dropped in recent years due to safety concerns and label restrictions.

Pill-based osteoporosis drugs, such as Fosamax and Zometa, have been available for more than a decade. But Roger Perlmutter, Amgen executive vice president, said many women can't tolerate those drugs or have trouble remembering to take a daily medication.

One out of every two women over age 50 will break a bone in their lifetime due to osteoporosis, which causes bones to become brittle, according to an FDA statement.

FDA approved the drug based on a 7,000-patient study conducted by Amgen that showed reduced vertebrae and hip fractures in postmenopausal women.

Common side effects with the drug included back pain, high cholesterol and urinary bladder infections.

Thousand Oaks, Calif.-based Amgen will be required to distribute patient guides explaining the drug's risks.

Global sales of osteoporosis treatments topped $8 billion last year, including hundreds of vitamin brands and drugs like GlaxoSmithKline PLC's Boniva and Merck & Co. Inc.'s Fosamax.

With eight other pills and injectable medicines on the market, analysts say Prolia's success likely will hinge on its price.

Prolia will cost $850 an injection, which the Amgen says is competitive with existing treatments.