What to do about excruciating trismus?

Depends: You have to get checked to see what is causing the trismus and then a treatment plan can be suggested.

It depends: Trismus is limited opening of the lower jaw, usually due to a problem in the jaw joint such as a displaced disc or possibly due to muscle spasm. If it's excruciating, you should seek treatment. In the meantime, stay on a very soft non-chewing diet, apply moist heat, and take otc non-steroidal anti-inflammatory drugs such as Ibuprofen or naproxen. See you local family doctor or an oral surgeon.