About 70 percent of the veterans in the trial served in Vietnam.

This drug has been used to treat PTSD symptoms. What happens when it fails a trial?

Thousands of people with post-traumatic stress disorder have taken the drug prazosin to ease the nightmares and disturbances that stalk their sleep.

Numerous studies have shown the drug to be effective at controlling those episodes. But a team of researchers from the Department of Veterans Affairs, seeking to collect more evidence, set out to study the sustained effectiveness of the treatment. They organized a large, lengthy, multisite trial — the most rigorous type of trial.