I agree, with anniemal...with as many health issues as you have my dearheart, I would be careful and trust your instincts. I wouldn't be surprised that if something isn't working for you, it may be giving you a bad reaction, or simply not working at all, and in that case I would say you have to switch.
I have to switch every so often because each antihistamine eventually stops working for me, but I don't think that's because I become allergic to them. This really was an "out of nowhere" thought.
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It would explain certain phenomena, but more likely explanations exist for those phenomena.