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ARTICLE TAKEN FROM: http://cheyennewest.com/antihistamines-for-dogs-cats/

The following is a list of options for antihistamines that are known to be effective in dogs & cats. The name of the active ingredient is listed, with the name of a well known brand in parentheses. Also listed are approximate doses for different weights, and any precautions.
CAUTION: Do not purchase antihistamines that are combination products
(multi-symptom, cold/flu medicines, decongestants, etc.), especially products that contain decongestants (often labeled with a D, as in “Zyrtec-D”). These products often contain a high level of stimulants that are toxic, even life-threatening, to animals. In addition, products containing acetaminophen (active ingredient in Tylenol) are deadly for cats.
1) Clemastine (formerly “Tavist”) – One of the more effective choices. Unfortunately, the over the counter form of Tavist containing the antihistamine “clemastine” has become unavailable. Therefore, we carry this antihistamine in our in-house pharmacy, in the 1.34mg strength.
Available in: 1.34mg tablets, 2.68mg tablets, and 0.67mg/5ml liquid
DOG WEIGHT (in pounds)                          Dose (tablet = 1.34mg tablet)
5-15                                                            (liquid) 1-2cc or ¼ tablet twice daily
15-25                                                             ½ tablet twice daily
25-50                                                              1 tablet twice daily
50-70                                                             1 or 2 tablets twice daily
Over 70                                                           2 tablets twice daily
CATS: ¼ to ½ tablet once daily (1.34mg tablets). Very effective in cats
May cause: sedation, hyperactivity, or diarrhea (in cats only).
Discuss with your veterinarian if your pet has: Prostatic hypertrophy (intact male dogs), urinary or GI obstruction, or severe heart disease
NOTE: Do not confuse this product with a similar product called “Tavist N.D,”
which is a different type of antihistamine, and is less effective.
2) Diphenhydramine (“Benadryl”)
Available in: 12.5mg children’s tablets, 25mg caplet, and 12.5mg/5ml liquid (liquid not recommended if it contains alcohol)
DOG WEIGHT (in pounds)                          Dose (tablet = 12.5mg, caplet = 25mg)
5-10                                                            ½ children’s tablet twice daily
10-15                                                             1 children’s tablet 2-3 times daily
15-35                                                             1 caplet 2-3 times daily
35-50                                                             1 or 2 caplets 2-3 times daily
50-80                                                             2 or 3 caplets 2-3 times daily
Over 80                                                          3 or 4 caplets 2-3 times daily
CATS: Generally not recommended. Clemastine, Chlorpheniramine and Cyproheptadine are more effective.
May cause: sedation, hyperactivity, diarrhea (rare), vomiting (rare), or loss of appetite (rare).
Discuss with your veterinarian if your pet has: glaucoma, GI or urinary obstruction, COPD (chronic lung disease), hyperthyroidism, seizure disorders, heart disease or high blood pressure.
NOTE: Do not use Diphenhydramine products that contain any other active ingredients (like acetaminophen). These other products are potentially toxic to your pet (especially cats!).
3) Chlorpheniramine (“Chlor Trimeton”)
Available in:  4mg tablets , and 2mg/5ml liquid
DOG WEIGHT (in pounds)                          Dose (based on 4mg tablet )
5-10                                                            1/4-1/2 tablet 2-3 times daily
10-20                                                            1/2 to 1 tablet 2-3 times daily
20-40                                                            1 to 2 tablets 2-3 times daily
40-60                                                            2 to 4 tablets 2-3 times daily
Over 60                                                         4 to 6 tablets 2-3 times daily
CATS: 1-4 mg per cat once or twice daily. Very effective in cats.
May cause: sedation or hyperactivity.
Discuss with your veterinarian if your pet has: glaucoma, high blood pressure, GI or urinary obstruction, hyperthyroidism, heart disease.
NOTE: Do not use chlorpheniramine products that contain any other active ingredients (like acetaminophen). These other products are potentially toxic to your pet (especially cats!).
4) Cetirazine (Zyrtec – Reactine)
This may be the best antihistamine for eosinophillic inflammation (one of the primary cell types associated with allergies). It is a formerly prescription-only medication that is now available over the counter.
Available in: 5mg and 10mg tablets
DOG WEIGHT (in pounds)                          Dose
Less than 15                                                One 5mg tablet once daily
15-39                                                          5mg twice daily, or 10mg once daily
Over 40                                                       10mg twice daily
CATS: 5mg ½ to 1 tablet 1 or 2 times daily.
May cause: Hyperexcitability in dogs
NOTE: Twice daily dosing is often needed.