I started with one Claritin per day, and after a short adjustment period I did fine with it.  Last night I added a second Claritin at night.  Now I am having problems with tachycardia.  I seem to start feeling normal about 4 hours before it is time to take the next pill.  The worst time the first 7 - 8 hours after I take it.  I had a very hard time sleeping last night with my heartrate up.  Did any of you have this problem?  Do you think I'll adjust in a few days?

Both my allergist and my cardiologist told me that Zyrtec is the safest antihistamine for heart issues. For me, starting Zyrtec stopped my long term tachycardia. Might be worth a try.