They are giving the drug just before having people describe their memories in several sessions. Adrenaline and its cousin noradrenaline, the same chemicals that trigger the fight-or-flight response, enhance the storage of fearful memories. Propranolol may block these actions on a cluster of nerve cells deep inside the brain.

Don't try this at home. Propranolol requires medical supervision to be used safely.

I spoke with Brunet about the state of research on propranolol-enhanced treatment. He and his colleagues administered propranolol 75 minutes before the reactivation of frightening memories in 40 patients. "It really did wonders," he told me. "After six sessions, 70 percent of patients no longer met the clinical criteria for PTSD." His recent study with Dr. Roger Pitman, a Harvard psychiatrist who directs the PTSD and Psychophysiology Laboratory at the Massachusetts General Hospital, and others appeared in the 2011 Journal of Clinical Psychopharmacology. Their patients were victims of trauma such as accidents, rape, and spousal abuse, and most had symptoms for years or decades. Symptom relief compared favorably to generally lengthier treatment with psychotherapy alone as reported in earlier studies.

Their current international study, funded by both Canadian and U.S. agencies, is designed to show whether this treatment works even when patients are randomly assigned to either propranolol or a placebo.

In an earlier placebo-controlled study, Pitman reported in Biological Psychiatry that patients treated with propranolol in the emergency room hours after a physical trauma, like an auto accident, were much less likely to show physical reactions to recalling their experience three months later. Here the treatment appeared to reduce the formation of new memories.

The PTSD burden on combat veterans is huge. One said that every night "I hear a woman scream ... there was a woman across the street from us, and we thought she had dynamite and was going to kill us. So I killed her," he told psychologist Paula J. Caplan for the Washington Post. It turned out that she did have dynamite and was planning to kill them. "But every night," he continued, "I hear her scream, because, well, I wasn't raised to kill."

Given the extent of the problem, there is some satisfaction that we can point to a location for disturbing memories. When medical students study the human brain, they identify a small cluster of nerve cells deep inside, called the amygdala from a Greek word based on its resemblance to an almond. This structure plays a key role in a fear network, storing memories of emotionally-charged experiences. Imagine it as a special memory stick for your computer where you save video clips of frightening events.

The location of the amygdala in the brain can be roughly visualized using your right hand with a forefinger curled around the thumb. The amygdala would be a small object at the tip of your thumb. The forefinger in front of the thumb represents the prefrontal cortex, the most advanced part of the human brain in primate evolution, located just behind the forehead.