Back in 2008, the Food and Drug Administration began reviewing bisphosphonate drugs for a possible connection between the osteoporosis drugs and atypical subtrochanteric femur fractures -- a break in the thigh bone just below the hip joint.

There's good news for bisphosphonate drugs, which includes Merck's (NYSE: MRK) Fosamax, Novartis' (NYSE: NVS) Reclast, sanofi-aventis (NYSE: SNY) and Procter & Gamble's (NYSE: PG) Actonel, and GlaxoSmithKline (NYSE: GSK) and Roche's Boniva. According to the FDA, "At this point, the data ... reviewed have not shown a clear connection" between bisphosphonate use and the breaks.

The key here is "at this point." The drugs are still under review, and like interest rates and dividends, the FDA's view is subject to change without warning.

Unfortunately for drugmakers, evidence continues to mount suggesting that the drugs make bones more brittle. In one study published this week, after taking the drug for longer than four years, the structural integrity of the bones started to decline. The drugs clearly help build bone mass, but the new bone may not be built as strongly as the natural kind.

Sales of the blockbusters would obviously be hurt if the FDA eventually decided that the drugs shouldn't be taken for extended periods of time.

Merck could care less about sales; revenue from its Fosamax franchise plummeted 49% last year as the drug started to see generic competition. But Fosamax is subject to lawsuits by almost 900 patients who claim Fosamax caused their jawbones to deteriorate. Any added side effects, especially if they're recognized by the FDA, won't help Merck's case.

Until more data comes in and the FDA bones up on it, investors will just have to take a wait and see attitude.

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Novartis is a Motley Fool Global Gains pick. Procter & Gamble is an Income Investor pick. Whether you like your companies big or small, dividend laden or with multibagger written all over them, we've got a newsletter for you.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool owns shares of GlaxoSmithKline and Procter & Gamble and has a disclosure policy.