That's going to be a harder and harder standard to enforce, as pharmaceuticals are increasingly being used routinely for "quality of life" purposes, rather than for survival, which used to be virtually the only reason. A lot of people taking Ritalin just do better in school or workplace with it, but are certainly not out of control or totally dysfunctionaly without it. Same for a lot of people using antidepressants and anti-anxiety meds. And the military should be comfortable with that, since they are experimenting with some heavy duty drugs to keep fighter pilots and Special Forces personnel awake and alert for unnatural lengths of time. If it's fine to use drugs to improve performance within the military, doing the same (and with much milder stuff) outside the military shouldn't be a bar to military service.
I think the problem is the logistics of getting all these happy pills to the troops in the middle of the desert, or whatever. For example, the medication I'm on, if I go off suddenly my head starts buzzing every few seconds, like a brain swoosh. No way would I be able to fight like that.