Having taken a course in the Law of Land Warfare from the Red Cross, and having just reviewed the Geneva Conventions, I can pretty confidently state that use of such drugs by your own forces would not be prohibited by the Geneva or Hague Conventions-- what you do with/to your own troops is not normally subject to those conventions. These agreements cover the misuse of medical insignia by combatants, the use of perfidy in war, the use of enemy uniforms, but I find nothing in them which would really control drugs or medicines you give to your own troops. I suspect that if you were trying to use psychoactive drugs on prisoners of war (or something similar), you would run afoul of the Geneva Conventions. Using psychoactive drugs in a combat situation on the enemy or on civilians (e.g. spreading LSD somehow on the battlefield) would likely violate the chemical weapons conventions (not really part of the Geneva Conventions), as this could be seen as chemical warfare. But the sanctioned use of drugs (e.g. Amphetamines by soldiers for military advantage) is clearly legal in this context.