By Victoria Gill
Science reporter, BBC News, San Diego
Progesterone protects neurons in the brain after an injury
Natural progesterone, the sex hormone used in the first contraceptive pills, is to be tested on patients with severe head injuries.
Scientists will begin a phase III clinical trial in March and say the drug could save patients' lives and reduce damage to their brains.
They announced the trial at the annual meeting of the American Association for the Advancement of Science.
It will involve 1,000 patients in 17 trauma centres across the US.
Dr David Wright, associate professor of emergency medicine at Emory University in Atlanta, will lead the trial.
Previous studies have shown that progesterone supports the normal development of neurons in the brain, and that the hormone has a protective effect on damaged brain tissue.
Dr Wright told BBC News: "Traumatic brain injury is a complex condition - there's swelling, and neuronal death and damage occurring all at the same time.
"The beauty of progesterone is that it seems to work on all of those things."
In earlier tests, the Emory University researchers found that progesterone reduced the risk of death in patients with brain injuries.
Dr Wright hopes that, following this trial, progesterone will become the first drug treatment in 30 years to be approved specifically for severe traumatic brain injury.
The active ingredient, natural progesterone, is very similar to that used in the first contraceptive pills. This has now been superseded by a synthetic progesterone known as progestin. But, for brain injury, only the natural hormone appears to have the desired protective effect.
During the trial, patients with blunt trauma head injuries will be given an infusion of natural progesterone that will last for four days. The hormone is extracted from wild yams.
"The dose is probably about three times what would be found in [the blood] of a female in the third trimester of pregnancy," Dr Wright explained.
The US Food and Drug Administration (FDA) has made a special allowance for the team to administer the drug without patients' consent - so it can be given as soon as possible and have the maximum protective effect.