Over the Counter medicine

The problem is all of the doses vary a GREAT deal.  Tavist, Claritin, Chlor-trimeton -- virtually ANY antihistamine can work for a dog BUT they are all dosed totally and completely different.  You can't apply the same dose rules to them as Benedryl.  They're based on metabolism but DIFFERENT.
 
Where a dog takes far more benedryl than a human, withTavist it's totally the opposite (and it's tough to give to a small dog because you have to cut that already tiny pill into so many parts!).  There's absolutely no comparison between the doses for any of them. 
 
Suggestions: 
 
The Pill Book Guide to Medications for your Dog and Cat -- Dell Reference (last published 199 - about $6.98 on Amazon.com.
 
Go to your public library and check out the Vet PDR (you won't be able to leave with it -- but bring a list of antihistamines you see on the shelf and how many mg are in each dose and you can then see what the dose would be.).  See things like Alavert probably can be used by dogs BUT I wouldn't have a clue what the dose would be and the Pill Book was pub in 1998. 
 
What you'll need to know for each is the actual drug name -- like Benedryl is diphenhydarmine and Tavist is Clemastine (I think I'm right there).  That's also going to tell you what the potential side effects are (like Tavist is more prone to side effects than Benedryl).
 
I'm picking on those two because I know them.  I've also used Chlor-trimeton ... but it's tricky.
 
See every single antihistamine actually serves a different purpose.  And most of us don't even know that.  I had a dermatologist (my own, not the dog's) who educated ME on antihistamines because according to him some antihistamines were formulated for different' things' -- like Benedryl is the 'best' (supposedly) for "skin itch" in humans (i.e., if you get skin itch -- like I do from basil -- Benedryl will do more than another antihistamine for THAT particular thing). 
 
The point is they react differently in different bodies (and species) -- so what works for me might not work for you.  So your though to try different ones is a GOOD thing -- you'll just need to do homework from books I don't have.  I have the Pill Book -- but it's not up to date enough for some drugs.  A vet PDR would be.
 
Honestly -- altho you may be a math whiz and able to calculate the dose yourself, I'd have your favorite pharmacist help double-check you.
 
And (I can't remember if I"ve ask this of you) -- have you checked the thyroid with Dr. Dodd's protocols?? (thru her website or Michigan State's??)  Made an incredible difference for Billy.
 
I also use a couple of Chinese herbals from my holistic vet that help balance things out.  They aren't specifically for "allergy" but rather skin issues that allergies tend to make happen (the little papules on the back and/or belly -- which are actually two separate things)
 
I'm also using homeopathics for him again this year (for he AND Kee Shu in fact) and having pretty good luck so far (looking for wood to knock on).  It is NOT convenient -- you ahve to give them like 4 times a day and you have to give several in concert with each other (or so I'm finding) so it's not cheap either.  BUT no side effects -- no sleepiness and NO kidney/organ damage. 
 
The other plus is they are a tad sweet ... and my two think they are mega treats!!  I have saved some little vials from other meds I took and I line them up and portion out the homeopathics so all I have to do is take a vial and dump them on the floor and Billy chomps them down like the finest candy. 
 
In short I do a LOT of stuff, but the difference is, THIS year his skin is holding and no incessant itching AND we are having the *absolute worst* allergy year in Florida history apparently.  So I feel like I'm doing something right.

Hey thanks callie. I guess I will have to do some more research before trying different antihistimes on Jetta.
She just finished her 2 months on z/d with no improvement, so I am really discouraged because nothing seems to work. I am now just trying to strenghten her immune system with a healthy diet, fish oil, and vit e. Also, I am bathing her whenever possible. But the itch does not stop!![&o]
 
I have not done the thryroid test yet but that I will definately bring it up next time we are at the vets. She just doesn't seem to have the symptoms of it though. I read somewhere that hot spots are not signs of thyroid issues, fur loss is. She has all her fur, but she has a bunch of hot spots! But then that source could have been wrong, i don't know...

Here is a link that gives dosages for dogs for several over the counter antihistamines according to weight ;
 
         [linkhttp://www.utskinvet.org/pdf/antihists.pdf]http://www.utskinvet.org/pdf/antihists.pdf[/link]
 
    A dog who is hypothyroid can have hair loss along both sides of the body. They can also gain weight easily, be lethargic , and have a poor appetite as well as a poor immune system.
   

That's kind of backwards.  Thyroid problems don't 'cause' skin problems -- they simply set the stage for other things to CAUSE the skin problems.  Like allergies.  So you help the thyroid (IF a problem) and that simply makes it more possible to be able to treat the allergies successfully.  It doesn't make them go away.
 
One of the things that has really helped us, is the traditional Chinese veterinary medicine -- there is usually internal 'heat' (which is oriental terminology and just doesn't 'translate' really) which the Chinese herbals really help keep under control.  The point being -- you can't stop the dog from being allergic, but you can treat the skin problems the histamine response causes.  By actually treating those rather than just pumping antibiotics into the body.  The skin problems then set the body up *for* infection.  The acupuncture helps a lot too. 
 
I've had really good luck referring folks to the Chi Institute website --http://www.chi-institute.com

Hey thanks Jessiesmom!! That chart is perfect! She is on an offbrand of hydroxyzine right now, and she has already had benadryl and clemastine, I guess i could try amitriptyline? Oh this is so frustrating.....

Amytriptaline is NOT an antihistamine.  It's a pain management & anti-depressant drug.

I used to have a lab with bad seasonal allergies and after trying several different antihistamines he was on amytriptaline - prescribed by a veterinary dermatologist.  I can't remember the uses of the drug in humans but in dogs it is an antihistamine and it worked for several years for my dog.
 
To the OP, have you gone to a dog dermatologist?  I went round and round with my dog until I went to one and finally found something that worked.
 
 

  Doxepin is used as an antidepressant in people too but my veterinary dermatologist prescribed it for Jessie while she was begining the immunotherapy ; it worked better than Benedryl.

No I haven't gone to a vet derm-- there isn't one around here.
 
Whoa I didn't know it was an antidepressant. I wonder why it can be used for allergies? Doesn't make sense....

I was curious myself, so I googled it and found this:
 
Other uses for this medicine[linkhttp://www.nlm.nih.gov/medlineplus/druginfo/medmaster/a682388.html#skip]Return to top[/link]
Amitriptyline is also sometimes used to treat chronic pain, eating disorders, and certain skin problems. Talk to your doctor about the possible risks of using this medication for your condition.
This medication may be prescribed for other uses; ask your doctor or pharmacist for more information.

It's also worth noting that humans and dogs do not always process medications the same - so something like Amytriptaline which has a pretty specific use in humans works in another way on dogs.  I would be very cautious about using human medications on dogs without knowing for certain what the effect would be on them.  To the OP, maybe you could ask your vet for a referral to another vet that might have more experience in dermatology, not necessarily a dermatologist though.  I know that they are pretty few and far between. 

Exactly -- you don't want to just willy nilly use a human med and not KNOW FOR SURE it can be used on a dog. 
 
For ex:  hydroxyzine -- it's both an anti-histamine and  anti-depressant BUT it was taken off the market years ago for humans because it can actually act AS a depressant for some people!   Works well for antihistamine in some dogs.
 
Tylenol -- helps with pain in humans, POISON to dogs.
 
I wasn't aware amytrip was ever used for an anti-histamine -- I took it myself years ago as a remedy for migraines.  One pill will make me sleep for weeks, but my ex used to take 3-4 at a time.  go figure.