Chlorpheniramine/
Pheniramine
Diphenhydramine
First
Generation H1receptor
antagonists,
ethanolaminederivative
antihistamine
Clemastine
Drug Class
First
Generation H1receptor
antagonists,
Alkaline
Antihistamine
First
Generation H1receptor
antagonists,
ethanolaminederivative
antihistamine
MOA
Antihistamines
competitively inhibit
histamine at H1 receptor
sites. They do not inactivate
or prevent the release of
histamine, but can prevent
histamine's action on the
cell. Besides their
antihistaminic activity, these
agents all have varying
degrees of anticholinergic
and CNS activity (sedation).
Like other antihistamines,
diphenhydramine
competitively inhibits
histamine at H1 receptors. In
addition, it also has
substantial sedative,
anticholinergic, antitussive,
and antiemetic effects.
Like other H1-receptor
antihistamines, clemastine
acts by competing with
histamine for sites on H1receptor sites on effector
cells. They do not block
histamine release, but can
antagonize its effects.
Clemastine has greater
anticholinergic activity, but
less sedation than average.
Use
Antihistamines are used in
veterinary medicine to reduce or
help prevent histamine mediated
adverse effects.
Chlorpheniramine is one the
more commonly used
antihistamines in the cat for the
treatment of pruritus. It may also
be of benefit as a mild sedative
in small animals due to its CNS
depressant effects.
Side Effects
Most commonly seen adverse effects
are CNS depression (lethargy,
somnolence) and GI effects (diarrhea,
vomiting, anorexia). The sedative
effects of antihistamines may diminish
with time. Anticholinergic effects (dry
mouth, urinary retention) are a
possibility. Chlorpheniramine may
cause paradoxical excitement in cats.
Palatability is also an issue with this
drug and felines.
Drug Interactions
Increased sedation can
occur if chlorpheniramine
is combined with other
CNS depressant drugs.
Antihistamines may
partially counteract the
anticoagulation effects of
heparin or warfarin.
It’s used principally for its
antihistaminic effects. Its
sedative effects can be of benefit
in treating the agitation
associated with allergic
responses. Treatment and
prevention of motion sickness
and as an antiemetic in small
animals. It has also been
suggested for use as adjunctive
treatment of aseptic laminitis in
cattle.
Clemastine may be used for
symptomatic relief of
histamine1-related allergic
conditions
The most commonly seen adverse
effects are CNS depression (lethargy,
somnolence), and anticholinergic
effects (dry mouth, urinary retention).
The sedative effects of antihistamines
may diminish with time. GI effects
(diarrhea, vomiting, anorexia), are a
possibility. Diphenhydramine may
cause paradoxical excitement in cats.
The liquid formulation is very
distasteful.
Increased sedation can
occur if diphenhydramine
is combined with other
CNS depressant drugs.
Antihistamines may
partially counteract the
anticoagulation effects of
heparin or warfarin.
Diphenhydramine may
enhance the effects of
epinephrine.
Sedation, paradoxical hyperactivity and
anticholinergic effects (dryness of
mucous membranes, etc.). In cats,
diarrhea has been noted most
commonly; one cat reportedly
developed a fixed drug reaction while
on this medication.
Additive CNS depression
may be seen if combining
clemastine with other CNS
depressant medications,
such as barbiturates,
tranquilizers, etc.
Monoamine oxidase
inhibitors (including
furazolidone) may intensify
the anticholinergic effects
of clemastine.
Hydroxyzine
First
Generation H1receptor
antagonists,
piperazinederivative
antihistamine
Hydroxyzine acts by
competing with histamine
for sites on H1-receptor sites
on effector cells.
Antihistamines do not block
histamine release, but can
antagonize its effects. In
addition to its antihistaminic
effects, hydroxyzine
possesses anticholinergic,
sedative, tranquilizing,
antispasmodic, local
anesthetic, mild
bronchodilative, and
antiemetic activities.
Hydroxyzine is used principally
for its antihistaminic, antipruritic
and sedative/tranquilization
qualities, often in atopic patients.
The most likely adverse effect
associated with hydroxyzine is
sedation. In dogs, this is usually mild
and transient. Occasionally
antihistamines can cause a
hyperexcitability reaction. Dogs have
reportedly developed fine rapid
tremors, whole body tremors and
rarely, seizures while taking this drug.
Cats may develop polydipsia,
depression or behavioral changes while
on this medication.
Additive CNS depression
may be seen if combining
hydroxyzine with other
CNS depressant
medications, such as
barbiturates, tranquilizers,
etc. Additive
anticholinergic effects may
occur when hydroxyzine is
used concomitantly with
other anticholinergic
agents. Hydroxyzine may
inhibit or reverse the
vasopressor effects of
epinephrine. Use
norepinephrine or
metaraminol instead.
Trimeprazine/ Promeprazine
First
Generation H1receptor
antagonists,
phenothiazine
antihistamine
Trimeprazine has
antihistaminic, sedative,
antitussive and antipruritic
qualities. The veterinaryapproved product also has
prednisolone in its
formulation that provides
additional antiinflammatory effects.
Trimeprazine is used alone for
the treatment of pruritic
conditions, especially if induced
by allergic conditions.
For trimeprazine, possible adverse
reactions include: sedation, depression,
hypotension and extrapyramidal
reactions (rigidity, tremors, weakness,
restlessness, etc.).
Other CNS depressant
agents (barbiturates,
narcotics, anesthetics, etc.)
may cause additive CNS
depression if used with
phenothiazines. Quinidine
when given with
phenothiazines may cause
additive cardiac depression.
Antidiarrheal mixtures
(e.g., Kaolin/pectin,
bismuth subsalicylate
mixtures) and antacids may
cause reduced GI
absorption of oral
phenothiazines. Increased
blood levels of both drugs
may result if propranolol is
administered with
phenothiazines.
Phenothiazines block
alpha-adrenergic receptors;
if epinephrine is also given,
unopposed beta activity
causing vasodilation and
increased cardiac rate can
occur. Phenytoin
metabolism may be
decreased if given
concurrently with
phenothiazines.
Oxatomide
Second
Generation
antihistamine,
firstgeneration of
the piperazine
class.
In addition to its H1 receptor
antagonism, it also
possesses antiserotonergic
actions. Oxatomide lacks
any anticholinergic effects.
Astemizole
second
generation
antihistamine
Astemizole competitively
binds to histamine H1receptor sites in the
gastrointestinal tract, uterus,
blood vessels, and bronchial
muscle. This suppresses the
formation of edema and
pruritus (caused by
histamine). Astemizole does
not cross the blood-brain
barrier, and H1 receptor
binding is mostly in the
peripheral rather than
central nervous system
(CNS depression is thus
minimal). Astemizole may
also act on histamine H3
receptors, thereby producing
adverse effects.
A piperazine derivative, is a
sedating antihistamine that has
also been reported to have mastcell stabilising properties. It is
used for the symptomatic relief
of allergic conditions including
urticaria, rhinitis, and
conjunctivitis
It has anticholinergic and
antipruritic effects.
Acute dystonic reactions and long-lasting
impaired consciousness were associated
with oxatomide therapy in 6 children.1
Impaired consciousness varied from
lethargy and somnolence
to a clinical picture resembling encephalitis
and persisted for 2 days or more in 3
patients.
It has been reported that this drug
might prevent much of the muscle
wasting (atrophy) that occurs in
immobile, bedridden patients. An
experiment on a small number of mice
showed that astemizole blocked the
activity of a protein present in muscle
that is involved in muscle atrophy.
However the concerns for the drug's
longterm effects on the heart preclude
its routine use in humans for this
indication.
It has been withdrawn from
the market in most
countries because of rare
but potentially fatal
interactions with CYP3A4
enzyme inhibitors (e.g.
erythromycin, grapefruit
juice).
Terfenadine
second
generation
antihistamine
acts on blood vessels and
gastrointestinal and
respiratory systems by
competing with histamine
for peripheral H1-receptor
sites; decreases allergic
response by blocking
histamine
formerly used for the treatment
of allergic conditions. It was
superseded by fexofenadine in
the 1990s due to the risk of
cardiac arrhythmia caused by
QT interval prolongation.
cardiac arrhythmia caused by QT
interval prolongation.
Loratidine
second
generation
antihistamine,
closely
structurally
related to
tricyclic
antidepressants
such as
imipramine,
and distantly
related to the
atypical
antipsychotic
quetiapine
Loratadine is a tricyclic
antihistamine, which acts as
a selective inverse agonists
of peripheral histamine H1receptors
It helps to relieve sneezing,
runny nose, and itchy, watery
eyes. This medicine is used to
treat the symptoms of indoor and
outdoor allergies. It is also used
to treat itchy skin rash and hives.
As a 'non-sedating' antihistamine,
loratadine causes less sedation and
psychomotor retardation than the older
antihistamines because it penetrates the
blood brain barrier only to a slight
extent. Although drowsiness is rare at
the common 10 mg dose, patients
should, nevertheless, be advised that it
can occur and may affect performance
of skilled tasks (e.g., driving); excess
alcohol should be avoided, although an
interaction between alcohol and
loratadine has not been shown.Other
possible side-effects include headache
and antimuscarinic effects such as
urinary retention, dry mouth, blurred
vision, and gastrointestinal
disturbances.
interaction with other
medications such as
erythromycin, or foods like
grapefruit. The addition of,
or dosage change in, these
CYP3A4 inhibitors makes
it harder for the body to
metabolize and remove
terfenadine. In larger
plasma concentrations,
terfenadine may lead to
toxic effects on the heart's
rhythm (e.g. ventricular
tachycardia and torsades de
pointes).
Substances that act as
inhibitors of the CYP3A4
enzyme such as
ketoconazole,
erythromycin, cimetidine
and furanocoumarin
derivates (found in
grapefruit) lead to
increased plasma levels of
loratadine. This had no
clinically significant effects
in controlled trials.