I think there is no difference in using generic and brand ones, the effect is almost the same. Both have some side effects like a decreased level of consciousness, nausea, respiratory distress, etc. Almost every other allergy medicine has some side effects.

have tried both and prefer generic over brand 

you might consider breaking tablet in half or even 1/4 tab

as we age our bodies do not metabolize as fast and you might be experiencing a drug overload rather than an reaction

I've never noticed a difference

I have not noticed a significant difference