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Note that in Zyrtec, lactose monohydrate is the first listed inactive ingredient, meaning that it comprises the greatest percentage of the inactive ingredients. So there's a lot of it in there.
With the Claritin Redi-tabs, mannitol is the third (next to last) listed ingredient, indicating that it comprises a minor part of the inactive ingredients. But of course, that's because the sugar alcohols are very effective sweeteners, so less is required to provide the desired degree of sweetness.
Basically (as you are probably aware), there are two types of antihistamines, first generation, and second generation. You're already tried all of the second generation antihistamines (except Alavert, and it also contains mannitol), so the only remaining options are some of the first generation products.
First-Generation OTC Antihistamines
Brompheniramine (Dimetapp Cold and Allergy Elixir)
Chlorpheniramine (Chlor-Trimeton)
Dimenhydrinate (Dramamine)
Diphenhydramine (Benadryl Allergy, Nytol, Sominex)
Doxylamine (Vicks NyQuil, Alka-Seltzer Plus Night-Time Cold Medicine)
Second-Generation OTC Antihistamines
Loratadine (some brand names: Alavert, Claritin)
Cetirizine (one brand name: Zyrtec)
Fexofenadine (one brand name: Allegra)
Until it was removed from the OTC market (a few years ago), my favorite antihistamine was Chlor-Trimeton (chlorpheniramine maleate) in the 12 mg formulation. It was much more effective for me than any of the other antihistamines (for pollen allergies). It was effective for me for 24 hours. And it's generally less sedating than the other first generation antihistamines. Unfortunately, it contained lactose. The 4 mg (4-hour) version has always been available, and a 12 mg formulation is now available again. Unfortunately, they both contain lactose about mid-way down a long inactive ingredient list.
http://www.walgreens.com/store/c/chlor- ... 0015878e71
http://www.chlorphen12.com/about-chlorphen-12/
It's even available on Amazon:
http://www.amazon.com/Chlorphen-Chlorph ... pc_title_2
And look at this price (for pets only):
http://www.vetrxdirect.com/product/view ... 7AodiHAAGQ
I can't help but wonder what the ingredients area, because there's a possibility that it might not contain a sweetener.
To be honest, I'm not aware of any OTC antihistamine that does not contain either some form of lactose, or a sugar alcohol (usually mannitol). I would assume that there is always the option of trying to find a compounding pharmacy that will compound a batch for you, to eliminate any ingredients that you need to avoid. If you can't find a local pharmacy, there are compounding pharmacies that will ship the formulations to you.
However, in order to arrange for a custom formulation, you would presumably need a prescription from your doctor (listing the ingredients, and the reason why this formulation is necessary), to authorize the pharmacy to produce the item. I'm not certain whether this is a requirement for OTC products though, so you might check with the compounding pharmacy first, to see if they would do it, and if they will, whether a prescription is required for an otherwise OTC product. (I suspect a prescription will be required).
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