Ritalin and Allegra Together | ADHD Information

My daughter has been diagnosed with ADHD/ADD and ODD (Defiance Disorder) for about 2 years now.  Although, my family and myself knew something was wrong way before then.  She is now almost 12 years old.  For over 2.5 years she has been breaking out in hives when she becomes over active or stressed.  A dermatologist just prescribed Allegra and Zantac to try to prevent or lessen hives break outs.  Does anyone know if these medications have shown bad side effects together?  And has anyones child had these hives break out due to stress and over activity?  Thanks,  Christina