June 7 (Reuters) - A preliminary review by the U.S. Food and Drug Administration of Merck & Co.'s experimental drug to treat the most common cause of health care-associated infectious diarrhea questioned whether the drug's efficacy had been adequately demonstrated.

The review, posted on the FDA's website on Tuesday, comes two days ahead of a meeting of outside experts who will discuss the drug, bezlotoxumab, and recommend whether or not it should be approved. The agency typically follows the advice of its advisory panels.

(Reporting by Toni Clarke in Washington)