In the past month, the Food and Drug Administration has concluded that in some cases two types of drugs that were supposed to be preventing serious medical problems were, in fact, causing them.

One is bisphosphonates, which is widely used to prevent the fractures, especially of the hip and spine, that are common in people with osteoporosis. Those drugs, like Fosamax, Actonel and Boniva, will now have to carry labels saying they can lead to rare fractures of the thigh bone, a surprising new discovery that came after another surprise  that they can cause a rare degeneration of the jawbone.

The other is Avandia, which is widely prescribed for diabetics, whose disease puts them at risk for heart attacks and heart failure. Two-thirds of diabetics die of heart problems, and a main reason for taking drugs like Avandia is to protect them from that.

But now the F.D.A. and drug regulators in Europe are restricting Avandia’s use because it appears to increase heart risks.