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 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. The World Health Organization has developed an online tool, called FRAX, to help determine when treatment for deteriorating bones is appropriate. A preliminary version can be found at www.shef.ac.uk/FRAX/index.htm. 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 FRAX formula 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. It was a WHO panel financed by the pharmaceutical industry that in 1994 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 WHO panel said its definitions of osteopenia and osteoporosis were not intended to provide reference points for diagnoses, much less for prescribing drugs. But Watts and others warn that this is happening, as more drugs become available to treat thinning bones and drug companies pay forbone-density measuring devices in doctors' offices -- and drugstores, shopping malls and health clubs. 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, however, is whether and when to start taking bone-loss drugs, which can cause gastrointestinal and other problems. Merck's popular drug Fosamax is the subject of hundreds of lawsuits by patients who assert that it caused osteonecrosis of the jaw, a rare disease that breaks down the jawbone. (The company says there is no proof of cause and effect.) 