By KATE MURPHY


As people age, their bones lose density and they grow ever more vulnerable to osteoporosis, with its attendant risk of a disabling fracture. But how do you know just how vulnerable you are?

The question has been complicated by a relatively new diagnosis: osteopenia, or bone density that is below what is considered normal but not low enough to be considered osteoporosis.

Millions of people worldwide, most of them women, have been told they have osteopenia and should take drugs to inhibit bone loss. But the drugs carry risks, so many public health experts say the diagnosis often does more harm than good.

Now the World Health Organization has developed an online tool meant to help doctors and patients determine when treatment for deteriorating bones is appropriate.

A preliminary version of the tool, called FRAX, was released last year and can be found at www.shef.ac.uk/FRAX/index.htm. A revised version is to be released later this year.

But FRAX is proving almost as controversial as the diagnosis of osteopenia. While some experts applaud it for taking factors besides bone density into account, others say that the formula on which the tool is based is faulty and that the advised threshold for medication is too low.

“FRAX is coming from the same people who came up with osteopenia in the first place,” said Dr. Nelson Watts, director of Bone Health and Osteoporosis Center at the University of Cincinnati, who said the diagnosis unnecessarily frightened women and should be abolished.

In 1994, a W.H.O. panel financed by the pharmaceutical industry defined normal bone mass as that of an average 30-year-old woman. Because bone naturally deteriorates with age, anyone much older than 30 is likely to qualify for a diagnosis of osteopenia.

The W.H.O. panel said its definitions of osteopenia and osteoporosis were not intended to provide reference points for prescribing drugs. But Dr. Watts warns that this is what is happening, as more drugs become available to treat thinning bones.

Since 2003, annual sales of osteoporosis drugs have about doubled to $8.3 billion, according to Kalorama Information, a market research firm.

The main controversy still involves whether and when to start taking the drugs, whose side effects can include gastrointestinal and other problems.

“Clearly, doctors have been at fault,” said Dr. Ethel S. Siris, director of the Toni Stabile Osteoporosis Center at Columbia University in New York. “But women need to educate themselves about the risks” before consenting to treatment.