Pharmacology - Oral antacids used in veterinary medicine are generally relatively non-absorbable salts of aluminum, calcium or magnesium. Up to 20% of an oral dose of magnesium can be absorbed, however. Antacids decrease HCl concentrations in the GI. One gram of these compounds generally neutralize 20-35 mEq of acid (in vitro). Although the pH of the gastric fluid can rarely be brought to near-neutral conditions, at a pH of 3.3, 99% of all gastric acid is neutralized, thereby reducing gastric acid back-diffusion through the gastric mucosa and reducing the amount of acid presented to the duodenum. Pepsin proteolytic activity is also reduced by raising the pH and can be minimized 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 of esophagitis, gastric hyperacidity, peptic ulcer and gastritis. Because of difficulty in administration and the frequent dosing that is often required, and with the advent of the histamine-2 blocking agents (cimetidine, ranitidine, et al) and/or sucralfate, antacids have largely been relegated to adjunctive roles in therapy for these indications in foals and small animals. They still remain important 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 patients with renal disease. Some products have significant quantities of sodium or potassium and should be used 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 antacid therapy are constipation with aluminum- and calcium-containing antacids, and diarrhea or frequent loose stools with magnesium containing antacids. Many products contain both aluminum and magnesium 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 patient chronically 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 calcium can be absorbed from the gut resulting in hypercalcemia in susceptible patients. Calcium carbonate has also been implicated in causing a gastric acid rebound phenomena. Patients with significant renal impairment or dehydration and electrolyte imbalance can develop the milk-alkali syndrome. If the patient is receiving a low phosphate diet, hypophosphatemia can develop if the patient chronically receives calcium carbonate antacids.
In ruminants, alkalinization of the rumen may enhance the absorption ofammonia, histamine or other basic compounds.
Overdosage - See the Adverse Effects section above. If necessary, GI and electrolyte imbalances that 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 other drugs. The reader is referred to specific references (see bibliography) for more information on the clinical significance and the individual salt(s) that have been implicated in the listing below. As a general 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 by antacids. Antacids should not be administered within 1-2 hours of tetracycline dosing. Antacids can decrease 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 the following: dicumarol, flecainide, quinidine, and sympathomimetics. Aspirin absorption and also excretion can be enhanced when concomitantly administered with antacids. Use of sodium polystyrene sulfonate (Kayexalate®) with antacids, may decrease the potassium lowering effectiveness of the drug and in patients in renal failure may cause metabolic alkalosis.
For rumen overload syndrome:
a) For adult animals: Up to 1 gm/kg (MgOH) mixed in 2-3 gallons of warm water and given PO per tube. May repeat (use smaller doses) at 6-12 hour intervals. If the rumen has been evacuated, do not exceed 225 grams initially. Dehydration and systemic acidosis must be concomitantly corrected.
Calves: As above but use 1/8th-1/4th the amount. (Wass et al. 1986a)
As an antacid:
a) Aluminum hydroxide: 30 grams;
Calcium carbonate: 60 - 360 grams (Jenkins 1988)
For adjunctive gastroduodenal ulcer therapy in foals:
a) Aluminum/magnesium hydroxide suspension: 15 ml 4 times a day (Clark and Becht 1987)
Monitoring Parameters - Monitoring parameters are dependent upon the indication for the product and the salt used. Patients receiving high dose or chronic therapy should be monitored for electrolyte imbalances outlined above.
Client Information/FDA Approval Status - Oral antacids are available without prescription (OTC). Most products are labeled for use in humans. There are veterinary approved products for use in food animals.
Oral Boluses 27 grams of magnesium hydroxide, ginger 200 mg, capsicum 100 mg, methyl salicylate 56 mg
Oral Powder, each pound of powder contains: 350 grams of magnesium hydroxide, ginger 2.6 grams, capsicum 1.3 grams, methyl salicylate 56 mg
Rulax II® (OTC)
Human-Approved Products: The following is a list of some antacids available, it is not meant to be all inclusive.
Aluminum Carbonate, basic
Capsules, equivalent to dried aluminum hydroxide gel 608 mg or aluminum hydroxide 500 mg
Suspension, equivalent to aluminum hydroxide 400 mg/5ml
475 mg; Alu-Cap® (Riker)
500 mg; Dialume® (Armour)
320 mg/5 ml; Amphogel ® (Wyeth-Ayerst)
400 mg/5 ml; Aluminum Hydroxide Gel® (Roxane)
600 mg/5 ml; Alternagel® (Stuart), Aluminum Hydroxide Concentrated® (Roxane)
Oral Suspension (Milk of Magnesia ) ≈77.5 mg/gram
Aluminum Hydroxide and Magnesium Hydroxide
Suspension (Note: there are too many products and concentrations to list in this reference; a representative product is Maalox® Suspension (Rorer) which contains 225 mg aluminum hydroxide and 200 mg magnesium hydroxide per 5 ml.)
Other dosage forms that are available commercially include: tablets, chewable tablets, and aerosol foam suspension.
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