WASHINGTON (AP) _ The Food and Drug Administration is cracking down on the only drug sold for nighttime leg cramps.

Some 2 million Americans buy quinine sulfate every year, mostly for leg cramps, a pain that usually responds to walking, stretching or massage.

The FDA says quinine sulfate never had to prove it helps leg cramps because it came on the market before the agency required such proof. The drug can pose dangers, including a life-threatening drop in blood counts that has claimed the lives of at least 16 people, the FDA said.

The agency mailed letters to 44 companies last month ordering them to stop marketing the drug for leg cramps.

The agency had already decided to ban the over-the-counter version, effective Feb. 22. But a consumer group asked the FDA to extend the ban to the prescription version, which comes in the same dose.

On Wednesday, the FDA said it can't ban prescription quinine sulfate because it is occasionally prescribed for the handful of malaria cases the nation gets. Instead, manufacturers must take all mention of leg cramps off their products and advertising.