My allergist has a sign up in his office that says you must wait 20 minutes after getting allergy shots before leaving. This is a precaution in case you have a bad reaction to the shots. My advice...
My allergist put me on Xyzal (which has been described as "Son of Zyrtec") a few months ago. I had a week or so of mild drowsiness initially, but nothing that seriously interfered with my day to day...