Many companies in India and China make tranexamic acid. Pfizer , which makes an injectable form for hemophiliacs (and donated thousands of doses to the Crash-2 trial), declined to give sales figures or even discuss administering it to trauma patients because the Food and Drug Administration has not approved that use. A company spokeswoman declined to say whether Pfizer had applied for approval. (Doctors may prescribe approved drugs for “off-label” uses, but drugmakers cannot endorse off-label uses without F.D.A. permission.)

The drug is believed to block plasmin, an enzyme that dissolves blood clots.

New York City ’s public hospital trauma doctors “are excited about the possibilities and discussing the risks and benefits” said Ana Marengo, a spokeswoman for the city’s Health and Hospitals Corporation. A decision will be made “in a couple of months,” she said.

Spokesmen for Cook County Hospitals in Chicago , San Francisco General Hospital and Grady Memorial Hospital in Atlanta said they too were moving toward using it, probably within two months. Los Angeles County hospitals have no such plans yet, a spokesman said.

Crash-2, which showed that getting the drug within three hours reduced the risk of fatal hemorrhage by 30 percent, “was an amazing study,” said Dr. John B. Holcomb, chief of trauma surgery at the University of Texas Health Science Center in Houston , which does use the drug. “Twenty thousand patients, and it was done in some places that had no lab tests.”

The United States Army took note when the drug was used on its soldiers in British Army hospitals.

“So we had a dog in that fight,” said Dr. Todd E. Rasmussen, an Air Force colonel who is now deputy commander of the Army Institute of Surgical Research in San Antonio .

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American surgeons were skeptical, he said, until he led a follow-up study, called Matters, which looked at the fates of 896 British patients.

It found that severely wounded patients who got the drug survived twice as often as those who had not; that convinced his American colleagues.

Recent wars have taught combat surgeons many new lessons that later caught on in civilian emergency rooms, said Dr. David E. Lounsbury, a retired colonel and co-author of a 2008 Army textbook, “War Surgery in Afghanistan and Iraq: A Series of Cases, 2003-2007.”

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Traditional hemorrhage treatment — giving intravenous saline solution to restore blood pressure — actually killed patients, he said, because it diluted clotting factors. Army doctors switched to giving blood and plasma; then, in the mid-2000s, added recombinant factor VIIa, a very expensive new clotting drug. But use of it faded, he said, after some patients got life-threatening clots on evacuation flights.

Tranexamic acid was never even in his combat hospital’s pharmacy. “An old generic doesn’t have any hair-on-your-chest bravado, so we didn’t even take it to the battlefield,” Dr. Lounsbury said.

That the British pioneered it “makes complete sense to me,” he added. “I worked in their hospitals, and they did pretty much everything we did — but much more cheaply.”

Dr. Holcomb, in Houston, said his hospital, the only one he knew that was now using it, gives it only to patients whose blood fails a clotting test. He insists on the test, even though it adds half an hour, because he is skeptical of one Crash-2 conclusion: that there are no side effects.

“Every drug has side effects,” he said. “Even aspirin.”