Regular Zyrtec (not Zyrtec D) may make you a bit drowsy for the first few days but not anything like Sudafed or Benadryl. If you take the D, that has pseudoephedrine in it and will probably make you drowsy but should go away in a few days as well.
My kids don't get drowsy on Zyrtec at all. I don't get drowsy on it, but I do slightly on the D...I feel more mellow than tired though. My friend did get somewhat drowsy for the first two days, so it kind of depends on your body. HTH