Fosamax, a leading osteoporosis drug has been linked to a sudden and unexplained bone breaks in patients taking the medication.

Although the active ingredient is supposed to make bones stronger, some doctors believe it is responsible for many spontaneous fractures and breaks.

A news station in Charleston S.C. reported on a woman whose leg snapped while she was walking. X-rays showed a clean break through the middle of her thigh bone - an extremely rare type of fracture and coupled with the fact that she neither tripped nor fell prior to the break has made doctors curious.

Furthermore, it's not a typical break for a person with osteoporosis.

Fosamax is designed to stop osteoclasts - cells that re-absorb bone in the body. Meanwhile, osteoblasts, the cells that make bone, are supposed to keep working, thus preventing osteoporosis. However, tests have revealed that both sets of cells have ceased function in patients who have suffered from these sudden fractures.

The rare fractures have been dubbed "frozen bone" by Dr. Robert Bunning, a lead rheumatologist at the National Rehabilitation Hospital. The brittle bone is more susceptible to fractures, with many patients only feeling vague thigh pain before the fracture.

Merck and Co Inc, the makers of the drug, and denied a link between the drug and the fractures. Osteoporosis has been blamed for many of the breaks, although doctors have been quick to point out that most osteoporosis breaks occur in the hip area and usually occur after a fall.

The U.S. Food and Drug Administration says it is aware of the allegations against the drug and are currently researching the situation. However, this is not the first harmful side effect linked to Fosamax.

The drug has been shown to cause a painful medical condition called osteonecrosis (ONJ), or "dead jaw." Osteonecrosis is a slow death of bone tissue in the jaw that occurs because of poor blood supply to the bone. Side effects of ONJ include mouth swelling, loosening of teeth and exposed bone.

A New York federal judge refused to dismiss the lawsuits against Merck in early 2010 alleging the drug caused ONJ in an Indiana woman who took the medication for close to eight years. The Fosamax lawsuit is one of almost 900 that have been filed nationwide.

Although the trial is set for April 2010, Merck has already committed to a vigorous defense, claiming other factors were responsible for the patients' jaw problems. Until generic versions became available, Fosamax generated close to $3 billion in sales for the company.