Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

ANTIHISTAMINES
antagonists and have been used in the management of
pruritic dogs and cats.
Clinical applications
In veterinary medicine, H 1 -blockers are used most frequently
in the management of allergic pruritus in the
dog and cat. Despite this, a recent evidence-based review
has concluded that there is currently insufficient evidence
to recommend the use of this range of drugs in
the management of canine atopic dermatitis. H 1 -
blockers may also be used as adjunct therapy for the
management of systemic anaphylaxis, to control ongoing
mediator release following the acute therapy of circulatory
collapse (e.g. with adrenaline (epinephrine),
fluids).
Formulations and dose rates
Suggested dosage regimens for the use of H 1 -blockers in
the management of pruritus have been reviewed in an
excellent paper by Scott & Miller (1999), and these data
are reproduced here (Tables 11.2 and 11.3). Effective
antihistamine therapy is problematic in the dog and cat,
and the owner and veterinary surgeon must be prepared
to invest some time in optimizing a regimen for any
individual patient.
Table 11.2 Antihistamine dosage for the pruritic dog
(from Scott & Miller 1999)
Antihistamine Dose (mg/kg) Frequency of
administration
Amitriptyline 1–2 q.12 h
Astemizole 1 q.12–24 h
Azatadine 1 mg/dog q.24 h
Brompheniramine 0.5–2 q.12 h
Cetirizine 0.5–1 q.24 h
Chlorphenamine
0.2–2 q.8–12 h
(chlorpheniramine)
Clemastine 0.05–1.5 q.12 h
Clomipramine 1–3 q.24 h
Cyproheptadine 0.1–2 q.8–12 h
Dimenhydrinate 8 q.8 h
Diphenhydramine 1–4 q.8 h
Doxepin 0.5–1 q.8–12 h
Doxylamine 1–2 q.8 h
Hydroxyzine 2–7 q.8 h
Ketotifen 2–4 mg/dog q.12 h
Loratadine 0.5 q.24 h
Oxatomide 0.5–2 q.12 h
Promethazine 1–2.5 q.12 h
Pyrilamine 1–2 q.8–12 h
Terfenadine 0.25–10 q.12–24 h
Trimeprazine 0.5–5 q.8–12 h
Tripelennamine 1 q.12 h
Table 11.3 Antihistamine dosage for the pruritic cat
(from Scott & Miller 1999)
Antihistamine Dose (mg/kg) Frequency of
administration
Amitriptyline 5–10 q.12–24 h
Chlorphenamine
2–4 q.12–24 h
(chlorpheniramine)
Clemastine 0.68 q.12 h
Cyproheptadine 2 q.12 h
Diphenhydramine 2–4 q.12 h
Hydroxyzine 5–10 q.8–12 h
Oxatomide 15–30 q.12 h
Promethazine 5 q.24 h
Not every antihistamine will be effective in any one
patient, so several agents and dosage regimens may need
to be tested in order to achieve control of pruritus. This
may involve the testing of a number of different antihistamine
drugs in sequence, with each agent being
evaluated for clinical effect over a 7–14 day period.
Factors such as cost and frequency of dosing should be
considered in selection of an appropriate antihistamine
for any individual case.
The antihistamines that appear most efficacious in the
dog include oxatomide, clemastine, cyproheptadine and
amitriptyline (achieving control of pruritus in 16–33%
of cases) and in the cat chlorphenamine (chlorpheniramine),
oxatomide, clemastine and cyproheptadine are
reported to be clinically effective in 40–73% of published
cases. There are, however, few blinded, placebocontrolled
trials of these agents upon which to base
specific recommendations.
In the management of pruritus, antihistamines are
generally given in conjunction with glucocorticoid, and
when pruritus is controlled (generally within 5 days) the
glucocorticoid is withdrawn and antihistamine therapy
maintained to prevent recurrence of pruritus. There is
some evidence that the effect of antihistamines may be
enhanced by concurrent supplementation with essential
fatty acids.
Scott & Miller (1999) have recommended an initial
14-day trial of antihistamine, followed by withdrawal
of the drug to allow clinical relapse. When relapse
occurs, the antihistamine therapy is started again and is
continued for a 30-day period. There is no clinical
benefit to be derived from using H 2 -blockers in the
therapy of canine atopic dermatitis, and there is no
additive effect to be gained from administration of combined
H 1 - and H 2 -blockers to such patients.
In adjunct therapy for systemic anaphylaxis, administration
of diphenhydramine (0.5–1.0 mg/kg IV to a
total dose of 50 mg) has been reported following appropriate
management of circulatory collapse.
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