Nuclear blasts open portals to hell
Every time you use a nuke, you open a portal to another dimension and creatures from it are able to invade Earth. These creatures are not intimidated by the Square Cube Law and are therefore insanely big and strong. They don't communicate with humans and simply commit genocide whenever they come.
Now that could be weaponized - you could simply nuke an enemy and let the aliens wipe out any survivors anyway. But that doesn't work because the portals move over the surface of the Earth - you could bomb some place 12 time zones away from you and still have some kaijus stomping on your capital.
Alternatively, if you think everything will be ok as long as no one shoots first, make it so that the mere presence of nuclear weapons opens up those portals.