H2-receptor antagonists are
structural analogs of histamine, commonly used to treat GI ulcers, erosive
gastritis, esophagitis, and gastric reflux. They act at the
H2 receptors of parietal cells to competitively
inhibit histamine, reducing gastric acid secretions during basal conditions and
when stimulated by food, amino acids, pentagastrin, histamine, or insulin.
examples of this group, also commonly referred to as H2
blockers. These drugs are rapidly absorbed, reaching peak plasma concentrations
within 1–3 hr. Ranitidine is widely distributed throughout the body.
H2 blockers are primarily metabolized in the liver.
Nizatidine, famotidine, and ranitidine are excreted in the urine as metabolites
and unchanged drug, whereas cimetidine is eliminated in feces. The elimination
half-life for these drugs is short (~2.2 hr). Because cimetidine may inhibit the
hepatic microsomal enzyme system, ingestion of an H2
blocker may result in reduced metabolism of certain drugs, including β blockers,
calcium channel blockers, diazepam, metronidazole, and theophylline.
H2 blockers have a wide margin of
safety, with acute oral overdoses typically resulting in minor effects such as
vomiting, diarrhea, anorexia, and dry mouth. Serious adverse effects, such as
tremors, hypotension, and bradycardia, are more likely to occur with IV
H2-blocker overdoses. The minimum lethal dose of
famotidine in dogs is >2 g/kg, PO, and 300 mg/kg, IV. Single oral doses of
800 mg/kg of nizatidine in dogs were not lethal. Most exposures require only
monitoring for development of GI signs and supportive care, although massive
overdoses may also warrant decontamination.
Antacids come in pill and liquid forms and are frequently
used to treat GI upset. Common antacids include calcium carbonate, aluminum
hydroxide, and magnesium hydroxide (milk of magnesia). These agents are poorly
absorbed orally. Calcium- and aluminum-containing antacids generally cause
constipation, whereas magnesium-containing antacids tend to cause diarrhea. Some
products contain both aluminum and magnesium salts in an attempt to balance
their constipating and laxative effects. Acute single ingestion of calcium salts
may cause transient hypercalcemia but is unlikely to be associated with
significant systemic effects. Induction of emesis within 2–3 hr of exposure may
help prevent severe GI upset.
Last full review/revision August 2014 by Safdar A. Khan, DVM, MS, PhD, DABVT