So here's the low down with anti-histamines per my research. Keep in mind there are two items you must keep in mind in terms of drugs: efficacy & side effects. I'll examine both.
There are two generations: 1st and 2nd.
1st generation includes meds like diphenhydramine (Benadryl). This med, in terms of efficacy, is by far the best. Benadryl is an extremely powerful anti-histamine. Side effects, however, are just as strong. It has anti-cholinergic effects and can dry up sputum. Additionally, the med crosses the blood brain barrier and causes lots of drowsiness (a SUBJECTIVE measurement) and impairment (OBJECTIVE measurement). There is a difference between drowsiness and impairment.
2nd generation includes Claritin (loratadine), Clarinex (desloratadine), Zrtec (ceterizine) and Allegra (fexofenadine).
Claritin was 1st to market and was considered a miracle drug cuz it worked pretty well and didn't have those horrible side effects like drowsiness and impairment like Benadryl did. The makers dosed it at 1/2 the European dose, though, so that it wouldn't cause as much drowsiness. Many people don't feel the efficacy of Claritin unless they double the 10mg dose to 20mg. At 20mg, the Euro dose, it can cause drowsiness or impairment. But it does not have anti-cholinergic effects.
Claritin is metabolized by the liver into Clarinex (loradadine into desloradadine) so you can treat them as the same drug.
There are 2 head to head studies, 1 compariing Zyrtec to Claritin, 1 comparing Allegra to Claritin, showing that both Zyrtec and Allegra relieve allergy symptoms better than Claritin.
Zyrtec has more sedation & impairment side effects than Allegra & Claritin, as it readily crosses the blood brain barrier. It also has mild anti-cholinergic effects, but not as bad as Benadryl.
Allegra does not have anti-cholinergic affects nor does it cross the blood-brain barrier so sedation & impairment do not occure.
2 head to head studies, both studying patients with the most severe allergy symptoms, showed that Allegra & Zyrtec are equally as powerful.
So the winner in my mind, and what I take, is Allegra 180mg.
Hope this helps
Anti-histamines Benedryl (diphenhydramine), Claritin (loratadine), Clarinex (desloratadine), Allegra (fexofenadine), Zyrtec (ceterizine), Xyzal (levoceterizine)