Stimulants are widely used in the civilian population to treat attention deficit hyperactivity disorder because they increase focus and attention. Short of an unlikely epidemic of that disorder among our soldiers, the military almost certainly uses the stimulants to help fatigued and sleep-deprived troops stay alert and awake. (A spokesman for Tricare attributed the sharp rise to “the increased recognition and diagnosis of A.D.H.D. by medical providers.” However, while there is greater recognition of the disorder, the diagnoses are concentrated in children and adolescents.)

Stimulants do much more than keep troops awake. They can also strengthen learning. By causing the direct release of norepinephrine — a close chemical relative of adrenaline — in the brain, stimulants facilitate memory formation. Not surprisingly, emotionally arousing experiences — both positive and negative — also cause a surge of norepinephrine, which helps to create vivid, long-lasting memories. That’s why we tend to remember events that stir our feelings and learn best when we are a little anxious.

Since PTSD is basically a pathological form of learning known as fear conditioning, stimulants could plausibly increase the risk of getting the disorder.

The role of norepinephrine in the enhancement of memory was demonstrated in an elegant experiment led by Larry Cahill at the University of California, Irvine. He randomly gave a group of subjects either propranolol, a drug that blocks the effect of norepinephrine, or a placebo just before they heard one of two stories: an emotionally arousing one or a neutral one. He then tested their memory of the stories a week later and found that propranolol selectively impaired recall of the emotionally arousing story but not the neutral story. The clear implication of this study is that emotion raises norepinephrine, which then enhances memory. Block norepinephrine and you can impair emotional memory. With PTSD, a shocking combat situation elicits a hard-wired fear response — the flight-or-fight reaction — with intense emotional arousal and a surge of norepinephrine in the brain. This burns in the memory of the traumatic experience. It also promotes fear conditioning, a form of learning in which previously neutral stimuli in the environment — sights, sounds and smells, for example — become linked with a trauma. So, for a soldier injured in a bomb blast, anything like the sound of an explosion or the odor of burning is now a potent conditioned stimulus that can evoke the trauma and trigger symptoms of PTSD, like a flashback or startle reaction.

Because norepinephrine enhances emotional memory, a soldier taking a stimulant medication, which releases norepinephrine in the brain, could be at higher risk of becoming fear-conditioned and getting PTSD in the setting of trauma.