Accident victims with head injuries could be treated with the hormone progesterone to reduce brain damage in the future.
A major trial of over 1,000 patients will examine whether progesterone can help reduce brain damage in injured patients.
Small levels of the hormone progesterone are found in male and female brains and is thought to play a key role in the normal development of neurons.
But research suggests that progesterone also exerts protective effects on damaged brain tissue.
An earlier study of just 100 patients showed that injections of the hormone soon after a head injury were safe and may reduce the risk of death and long-term disability.
The new Phase III trial conducted at 17 medical centres around the US will enrol around 1,140 brain damaged patients over three to six years.
This kind of trial usually marks the final testing stage before a treatment is made widely available.
The ProTECT III study will be led by scientists from Emory University in Atlanta, Georgia, where the neuro-protective properties of progesterone were first discovered more than 25 years ago.