One of the main risks is that they are based in fluoride compounds. Fluoride is known to cause this type of reaction - that is "swiss cheese" bone structure that leads to brittle and weak bones.
Now it seems that medical pundits want you to be exposed to cancer causing ionizing radiation in x-ray diagnosis of the problem of rotting jaw bone, a serious side effect of these drugs.
In order to prevent reliance on the drugs that are commonly used in medical care for osteoporosis, we have developed a protocol that is based on natural treatment and people who choose it, after we provide them with the educational material that is required, seem to do very well with getting healthier bones.
X-rays Help Predict Permanent Bone Damage From Bisphosphonates
ScienceDaily (2009-05-12) -- Bisphosphonates have been found to place people at risk for developing osteonecrosis of the jaws (a rotting of the jaw bones). Dentists, as well as oncologists, are now using X-rays to detect "ghost sockets" in patients that take these drugs and when these sockets are found, it signals that the jawbone is not healing the right way. Early detection of these ghost sockets can help the patient avoid permanent damage to their jawbone. ... > read full article