The military is worried about all the drugs that soldiers are taking:
Abuse of pain pills by troops concerns Pentagon
Senator Jim Webb has made comparisons to his time in Vietnam, when soldiers were mostly getting hooked on heroin. Now it's prescription antidepressants and pain killers.
Your first reaction may be to wonder how things have changed... but I bet they haven't changed much at all. One of the things that most people don't know about the opiates - they are some of the best antidepressants known to man, even in this modern age of Prozac. They haven't been prescribed that way since the 1950's, because they're addictive (and somewhat dangerous).
Most likely war is a pretty depressing thing, and soldiers who can't get treatment one way will find it another way. So they take speed on the way in, and opiates on the way out.