Page 466 - Small Animal Internal Medicine, 6th Edition

438 PART III Digestive System Disorders

TABLE 28.3
VetBooks.ir Selected Antiemetic Drugs DOSAGE* COMMENTS

DRUG

Centrally Acting Drugs
Neurokinin-1 receptor antagonist
Maropitant (Cerenia) 1 mg/kg SC q24h (dogs or cats) Treat puppies 2-7 months old for 5 days;
2 mg/kg PO q24h (dogs) older animals can be treated
1 mg/kg PO q24h (cats) indefinitely
8 mg/kg PO q24h for up to 2 days for
motion sickness in dogs
4 mg/cat PO q24 h for vomiting due to
renal failure
Serotonin receptor antagonists
Ondansetron (Zofran) 0.1-0.2 mg/kg IV q8-24h Oral administration much less effective
0.5 mg/kg in resistant cases
Metoclopramide (Reglan) 0.25-0.5 mg/kg PO, IM, or IV q8-24h Can see CNS adverse effects; poorly
effective in cats
0.4 mg/kg loading dose, then 0.3 mg/kg/h Most useful when used as an “add on”
by CRI to maropitant or ondansetron; protect
from light when administering as CRI
Phenothiazine derivatives
Chlorpromazine (Thorazine) 0.3-0.5 mg/kg IM, IV, or SC q6-8h Watch for hypovolemia
Prochlorperazine (Compazine) 0.1-0.5 mg/kg IM q8-12h

IM, Intramuscularly; PO, orally; SC, subcutaneously.
*Dosages are for both dogs and cats unless otherwise specified.

poor oral bioavailability (food does not affect absorption) a dosage of 0.3-1.0 mg/kg/hr by constant rate infusion. In
but good absorption after SC administration. It is such an particular, metoclopramide may be used as an “add on” in
effective antiemetic that it will often prevent vomiting sec- difficult-to-control patients not responding to NK-1 and/or
ondary to foreign body obstruction, so it is important to try serotonin receptor antagonists.
to rule out such obstruction before administering this drug. Phenothiazine derivatives (e.g., prochlorperazine [Com-
Gastrointestinal perforations have occurred because success pazine]) are often effective but are seldom used now. They
with maropitant delayed diagnosis and removal of foreign inhibit the chemoreceptor trigger zone and, in higher doses,
bodies. It has analgesic effects for visceral pain. the medullary vomiting center. Antiemesis is usually achieved
Ondansetron (Zofran) and dolasetron (Anzemet) are at doses that do not produce marked sedation. However,
serotonin (5-hydroxytryptamine, 5-HT) receptor antago- these drugs may cause vasodilation and can decrease periph-
nists. They are good drugs but are typically less effective than eral perfusion in a dehydrated animal.
maropitant. Mirtazapine (primarily used as an appetite stim- Many other drugs have antiemetic effects. Mu-antagonist
ulant) may also have some antiemetic effects due to its antag- narcotics (e.g., fentanyl, morphine, methadone) may cause
onism of 5-HT. vomiting initially, but vomiting is usually inhibited once the
Metoclopramide (Reglan) is a less effective antiemetic drug penetrates to the medullary vomiting center. Butorpha-
than the NK-1 and serotonin receptor antagonists (espe- nol has some efficacy as an antiemetic and is sometimes used
cially in cats). It inhibits the chemoreceptor trigger zone in patients undergoing chemotherapy.
and increases gastric tone and peristalsis. Rarely, animals
show unusual behavior (e.g., excitation) after administra-
tion. The drug is excreted in the urine, and severe renal ANTACID DRUGS
failure makes adverse effects more likely. It rarely worsens
vomiting, perhaps because it causes excessive gastric con- Antacid drugs (Table 28.4) lessen gastric acidity. Although
tractions. The liquid form of metoclopramide given orally is they are not antiemetics, they sometimes seemingly have an
often not palatable to cats. Because of its prokinetic activity, “antidyspeptic” effect due to diminishing gastric hyperacidity.
the drug is contraindicated in animals with gastric or duo- Antacids, which titrate gastric acidity, are over-the-coun-
denal obstruction. Metoclopramide may be more effective ter preparations that are typically of limited efficacy. Com-
in animals with severe vomiting if given intravenously at pounds containing aluminum or magnesium tend to be