US regulators said Tuesday that Xarelto, a Bayer-made drug approved in July for preventing blood clots, is so far unproven for a new proposed use as a stroke preventer.
"There is a lack of substantial evidence that rivaroxaban will have its desired effect when used as recommended in labeling," said documents released by the US Food and Drug Administration.
An advisory committee to the FDA would further discuss the latest findings on Thursday.
"This review posted today has to do with the application for the new use and at this point does not impact the already-approved use," said FDA spokeswoman Sandy Walsh.
Bayer's stock plunged nearly 12 percent by midday in US trading.