Google Edgewood Arsenal and the BZ munition.

Long story short - just like with gay bombs and all these other sorts of Cold War esoterica - is that humans are really complicated, and war is even more complicated. Turns out making people gay or psychotic - even if the chemicals did work at doing this, which is dubious given their tenuous understanding of what "gay" or "psychosis" was - doesn't actually reduce their warfighting capability. Sometimes, it even made a tenable tactical situation much worse.

Now, when it came to making people think they were going crazy, that's where these substances really started to find some traction, but that wasn't a military application. More the territory of the CIA headshrinkers.

I would guess you wouldn't even need to make it a weapon. Just airdrop ready-to-use drugs across their positions. Enough bored and/or shell shocked soldiers will self administer.

Along that line what if you dropped cocaine or heroine over the enemy country? These things are cheap to produce.

What about crack? Even cheaper, and you could distribute it in the inner cities (or wherever it made sense) to control a target population

400M fentanyl pills seized in US last year, 70% of them lethal.