There are two generations of antihistamines on the market.

The first generation includes the ones that cause drowsiness. These enter brain and have central effects besides the antihistamine ones. They are derived chemically from the phenothiazines (antipsychotics). This is why some like Vistaril, are used as sedatives. Dramamine orig formula, Benadryl, ChlorTrimeton, and Antivert (meclizine for dizziness) and Atarax belong to this class. All of these have multiple effects on various neurotransmitters, and are used for other things besides allergies. Benadryl is used for some forms of dyskinesia in fact.

Histamine is a neurotransmitter in the brain...it precedes release of serotonin and other neurotransmitters, and is considered a signaling molecule. So the first generation antihistamines affect many things, because they block histamine's job of initiating actions.

Claritin and Allegra, are second generation antihistamines, and much less potent or useful for allergies. And they do not cross into the brain, and have limited usefulness. In fact there are studies showing Claritin is equal to placebo in effectiveness!

Zyrtec (cetirizine is sort is inbetween the generations. It is a cousin of Atarax which is considered the most potent antihistamine for itching. It has some sedative actions but not as strong as Atarax.

Xyzal (levocetirizine) is related to Zyrtec and does not cross into the brain at all.

As the affinity to cross into the brain, is lost, so are the other actions that are common to the first generation drugs lost. So one would not expect Allegra or Claritin or Xyzal to work on drying things up like Benadryl does. If the runny nose is a Mast cell allergic response, they may work. But for all other causes, they would be ineffective.