I am a long term user of anthistamines and use antihistamines for at least 2 weeks of the month before my period when everything just itches, although do tend to reduce the dosage by splitting my tablets in 1/2 or if it is easy into quarters.
One of the things I observed is that I tend to be more forgetful and wondered if this was a sign of dementia. Had also noticed when am off antihistamine, the cognition improved and my memory was far better.
Interesting to read this article that appears to confirm my observations:
Here is another article you might want to look at if you use Anthistamines:
Remember to drink more water (find using a bottle helps me) and to eat more greens (thankfully I grow my own)