TEL AVIV, Israel, Dec. 2 -- An injection of cortisol shortly after exposure to a traumatic event may prevent the onset of post-traumatic stress disorder, researchers in Israel said.

Joseph Zohar of the Sackler Medical School at Tel Aviv University said he is taking his animal model findings to the U. S. National Institute of Health and hopes to start clinical trials within the next year.

Currently, by definition, a diagnosis of PTSD is made only after an individual has been living with an acute stress reaction for one month. However, by then it may be too late to counteract the syndrome, Zohar said.

"Ten percent to 20 percent of all individuals exposed to trauma develop PTSD," Zohar said in a statement. "The challenge is to try to prevent or reduce these numbers. Until now, the clinical and research focus has been on treating PTSD once it developed. We propose to shift the focus to prevention."

Normally, the production of cortisol, a stress hormone, increases immediately after the trauma, but with time returns to normal levels. In those who are diagnosed with PTSD, however, the body's hormonal system is dysfunctional.

The study, published in the journal Biological Psychiatry, found in an animal model that a high dose of corticosterone -- a steroid hormone -- when given immediately after the stress event, reduces the effect of trauma in mice.