Is this a thing that requires testing? Do they have units run drills on all common recreational drugs? I could see some merit in evaluating stimulants, but what is the expected gain from most anything else?

They considered using L.S.D. as an aerosol on enemy troops, but IIRC there were problems with using it, don't remember exactly what. It would also incapacitate friendly troops or some agreement was signed forbidding its usage. Long time ago since I went down this rabbit hole last time...

Right, but there are known agents that are incredibly legal. I am still trying to imagine the scenario in which making the enemy ineffective was a superior option to dead. I guess the non lethal option lowers the risk of friendly fire?

If nothing else, the double edged sword application of chemical weapons was well known at this point.

The ideal scenario for a weapon is immediate, non-lethal, incapacitation. The holy grail of weapons technology is the stun setting on the Star Trek phaser.

Making an incapacitated enemy dead: Easy.

Making a dead civilian/ally un-dead: Necromancy.

If it were used as an environmental toxin, would it cause the soldiers to begin shooting at each other?

No, they might start empathising with each other.