For many of us, anti-histamines like diphenhydramine and chlorpheniramine have been essential for our treatment as well as anticholinergic drugs like oxybutynin and glycopyrrolate. Acetylcholine and something called 'cholinergic crisis' is relevant. I recently did some blood work to determine my cholinesterase levels and they are low which supports, at least in part, that for some of us, low cholinesterase is a key component of our POIS symptoms.
The blood testing is very inexpensive and is totally work doing as it may help guide your discovery/treatment. I ordered myself through Ulta Lab Tests and just went to one of the local labs in my area. Here is the link
https://www.ultalabtests.com/testing/search?__RequestVerificationToken=D3sk4rUwi28j0Lq6d43ylFTnJ_2N0cnY_J9o6wTEiAIkCBuIC-hOB7lCh2_8C9X4FC-V4kTcAsjaefTmxaB4mYfLXJA1&ItemName=cholinesteraseHopefully some of you will find benefit by doing this testing. Of course, this may not be the answer to everything, but it can be very useful. Other testing of value can be thyroid, adrenal, inflammatory markers, pathogens, viral, gut dysbiosis, some of which can be tested with some assurance of validity while others are more fringe, but still potentially of value.
There are also nootropic supplements that can be of value, in my case with acetylcholine being a problem (piracetam, bromantane, noopept, phenylpiracetam, centrophenoxine, etc.) Hope this is helpful for at least some of you.