My allergist gave my wife and I a plan that works best for our specific allergies.

I use Allegra-D (one tablet twice daily).

My wife uses Allegra-D during the day to prevent drowsiness and Zyrtec-D at night, because it knocks her out.

I have really BAD allergies, nothing life threatening though, but I can take the mild sedation of the medication (histamine blocker) versus the extreme fatigue and "wishy-washy" feeling I have from the huge release of histamines, which I understand is the body's defense against the allergen(s).

Depending on the dosage and how often you can take them based on your prescription, I would suggest a Zyrtec (especially the all-day dose) before bed so the heavy sedation can take place while sleeping and it will taper off during the day, otherwise give it about three weeks and then tell your doctor and see if there is another that will have a less severe sedating effect. You may have to experiment for a while, but I've heard it said often that these medicines are like vitamins, "They take a while to feel the benefit(s) and over time you will lose sight of their true value, but you WILL see just how well they really did work for you immediately if you stop taking them."