What should I do to minimize adverse reaction I had to allegra-d?

Need more info. More information is needed before one can provide the advice on how to minimize the adverse reaction. Specifically what reaction was experienced? How long did it last? And has it happened before?
Which of 2 drugs? Allegra (fexofenadine) d contains 2 drugs - an antihistamine & a decongestant. If reaction not severe - for example Allegra (fexofenadine)-D kept you awake at night - you can try this. Allegra (fexofenadine) only contains antihistamine. With your doctor's approval & supervision take only allegra (fexofenadine). If symptoms return the cause is the histamine. If they don't the cause is the decongestant. Now you & your doctor can decide what to do.