The Food and Drug Administration (search) has approved a monthly version of osteoporosis drug Boniva, Roche Holding AG (search) and GlaxoSmithKline Plc (GSK) said on Friday.

A daily version of the pill was first approved by the FDA in 2003, but the companies never launched the product as they explored a more convenient dosage for the medicine to treat brittle bones.

Boniva, known in Europe as Bonviva, is part of a popular class of drugs known as bisphosphonates that aim to treat post-menopausal women with osteoporosis (search).

Osteoporosis is most common in older women when levels of estrogen, a hormone, drop. It occurs in about one in three elderly women and one in 12 older men.

Most of the drug's rivals -- including Merck & Co Inc.'s (MRK) Fosamax (search) and Sanofi-Aventis SA /Procter & Gamble Co's (PG) Actonel (search) -- need to be taken either daily or weekly.

While these drugs are effective at preventing bone loss, they are tricky to use because they have to be taken in the morning on an empty stomach and patients must remain upright for 30 to 45 minutes.

A monthly pill would minimize those aggravations. Likewise, the launch of once-monthly Boniva could ultimately shake up the multibillion-dollar osteoporosis market now dominated by the rival once-a-week treatments.

Fosamax sold $3.2 billion worldwide in 2004 while Actonel raked in about $1.5 billion..

Other companies, including Novartis AG and Amgen Inc., are working on even longer-lasting products that would be given once or twice a year.

Mike Ward, a pharmaceuticals analyst at Code Securities, says the trend is clearly toward less frequent dosing, with Fosamax and Actonel having replaced previous daily treatments.

"There are difficulties taking these tablets and the weeklies have taken well over 90 percent of the market because of the benefits they offer," he said.

That trend could position Roche and Glaxo well when they roll out the drug.

Analysts' average forecast for Boniva/Bonviva sales by 2009 is $783 million, according to independent consultancy Evaluate.

However, Kepler Equities analyst Denise Anderson said she believes the market may be underestimating the product's potential, which could also benefit from Glaxo's marketing clout.

Roche and Glaxo in December sought U.S. approval for an intravenous form of Boniva to be given once every three months and expect an FDA decision in late 2005.