ANTACIDS, ORAL

Pharmacology - ANTACIDS, ORAL

Oral antacids used in veterinary medicine are generally relatively non-absorbablesalts of aluminum, calcium or magnesium. Up to 20% of an oral dose of magnesium can beabsorbed, however. Antacids decrease HCl concentrations in the GI. One gram of these compoundsgenerally neutralize 20-35 mEq of acid (in vitro). Although the pH of the gastric fluid can rarely bebrought to near-neutral conditions, at a pH of 3.3, 99% of all gastric acid is neutralized, therebyreducing gastric acid back-diffusion through the gastric mucosa and reducing the amount of acidpresented to the duodenum. Pepsin proteolytic activity is also reduced by raising the pH and can beminimized if the pH of the gastric contents can be increased to >4.
Uses, Indications - Antacids have been used in veterinary medicine for the adjunctive treatment ofesophagitis, gastric hyperacidity, peptic ulcer and gastritis. Because of difficulty in administrationand the frequent dosing that is often required, and with the advent of the histamine-2 blockingagents (cimetidine, ranitidine, et al) and/or sucralfate, antacids have largely been relegated toadjunctive roles in therapy for these indications in foals and small animals. They still remainimportant in reducing hyperphosphatemia in patients with renal failure.
In ruminants, magnesium hydroxide is used to increase rumen pH and as a laxative in the treatment of rumen overload syndrome (aka acute rumen engorgement, rumen acidosis, grain overload, engorgement toxemia, rumen impaction).
Contraindications/Precautions - Magnesium-containing antacids are contraindicated in patientswith renal disease. Some products have significant quantities of sodium or potassium and should beused cautiously in patients who should have these electrolytes restricted in their diet. Aluminum-containing antacids may inhibit gastric emptying; use cautiously in patients with gastric outlet obstruction.

Adverse Effects, Warnings

In monogastric animals, the most common side effects of antacidtherapy are constipation with aluminum- and calcium-containing antacids, and diarrhea or frequentloose stools with magnesium containing antacids. Many products contain both aluminum andmagnesium salts in the attempt to balance the constipating and laxative actions of the other.
If the patient is receiving a low phosphate diet, hypophosphatemia can develop if the patientchronically receives aluminum antacids. Magnesium-containing antacids can cause hypermagnesemia in patients with severe renal insufficiency.
If administering calcium carbonate in high doses or chronically, significant quantities of calciumcan be absorbed from the gut resulting in hypercalcemia in susceptible patients. Calcium carbonatehas also been implicated in causing a gastric acid rebound phenomena. Patients with significantrenal impairment or dehydration and electrolyte imbalance can develop the milk-alkali syndrome. Ifthe patient is receiving a low phosphate diet, hypophosphatemia can develop if the patientchronically receives calcium carbonate antacids.
In ruminants, alkalinization of the rumen may enhance the absorption ofammonia, histamine orother basic compounds.
Overdosage - See the Adverse Effects section above. If necessary, GI and electrolyte imbalancesthat can occur with chronic or acute overdose should be treated symptomatically.

Drug Interactions

By altering GI transit time, stomach pH, or by chelation, all orally administered non-absorbable antacids can affect the rate and potentially the extent of absorption of otherdrugs. The reader is referred to specific references (see bibliography) for more information on theclinical significance and the individual salt(s) that have been implicated in the listing below. As ageneral guideline, it is best not to give antacids within 1-2 hours of other oral medications.
Orally administered tetracycline products can be chelated and prevented from being absorbed byantacids. Antacids should not be administered within 1-2 hours of tetracycline dosing. Antacids candecrease the amount absorbed or the pharmacologic effect of: chlordiazepoxide, captopril, chloroquine, cimetidine, corticosteroids, digoxin, iron salts, indomethicin, isoniazid(aluminum antacids only), ketoconazole, nitrofurantoin, pancrelipase, penicillamine, phenothiazines, phenytoin, ranitidine, and valproic acid.
Increased absorption or pharmacologic effect may occur when antacids are administered with thefollowing: dicumarol, flecainide, quinidine, and sympathomimetics. Aspirin absorption andalso excretion can be enhanced when concomitantly administered with antacids.
Use of sodium polystyrene sulfonate (Kayexalate®) with antacids, may decrease the potassiumlowering effectiveness of the drug and in patients in renal failure may cause metabolic alkalosis.