Well, this may be the answer to why my mom has AD. She seldom had a pocket that didn't hold an OTC sinus allergy inhaler. My dad got frequent hay fever but I don't know if he took much in the way of antihistamines for it. He didn't have AD but he passed away in December after a little over a month of heartbeat that was too fast and they couldn't give him the usual blood thinner because when they tried it, it revealed a bleed in his brain. He was 89 and while he did have three arteries bypassed and several stents placed, he lived longer and a more normal life than we ever expected.
The point about that being that while the antihistamines might bring on AD for some, not everyone reacts that way. It's all such a mystery. It drives me nuts.