Zyrtec is certirizine which is a second generation anti-histamine that only causes very mild drowsiness due to only crossing the blood brain barrier very slightly. But maybe these anti-histamines just don't agree with you? Plus the meds commonly given for dp/dr act differently then anti-histamines. Although certain anti-psychotics seroquel for one and certain anti-depressants remeron (mirtazapine) for one are strong H1 antagonists thus they do have strong anti-histiminic side effects. Drugs such as the benzodiazepines, anti-convulsants, ssri's and many more don't act as anti-histamines at all.
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