Chemistry - Magnesium cation containing solutions of magnesium citrate, magnesium hydroxide, or magnesium sulfate act as saline laxatives. Magnesium citrate solutions contain 4.71 mEq of magnesium per 5 ml. Magnesium hydroxide contains 34.3 mEq of magnesium per gram and milk of magnesia contains 13.66 mEq per 5 ml. One gram of magnesium sulfate (epsom salt) contains approximately 8.1 mEq of magnesium. Solutions containing phosphate anions also act as saline laxatives. These solutions generally contain monobasic and/or dibasic sodium phosphate. Polyethylene glycol 3350 is a non-absorbable compound that acts as an osmotic agent.
Storage/Stability/Compatibility - Magnesium citrate solutions should be stored at 2-30°C. Store Milk of Magnesia at temperatures less than 35°C, but do not freeze. PEG 3350 reconstituted (from powder by the pharmacy, client, clinic, etc.) solutions should be kept refrigerated and used within 24 hours.
Pharmacology - Although unproven, it is commonly believed that the hyperosmotic effect of the poorly absorbed magnesium cation or phosphate anion causes water retention, stimulates stretch receptors and enhances peristalsis in the small intestine and colon. Recent data, however, suggests that magnesium ions may directly decrease transit times and increase cholecystokinin release.
Polyethylene glycol 3350 is a non-absorbable compound that acts as an osmotic agent. By adding sodium sulfate as the primary sodium source, sodium absorption is minimized. Other electrolytes (bicarbonate, potassium and chloride) are also added so that no net change occurs with either absorption or secretion of electrolytes or water in the gut.
Uses/Indications - The saline laxatives are used for their cathartic action to relieve constipation. They are also used to reduce intestinal transit time thereby reducing the absorption of orally ingested toxicants. Polyethylene glycol 3350 balanced electrolyte solutions are used to evacuate the colon prior to intestinal examination or surgery.
Pharmacokinetics - While it is unknown how much sodium or phosphate is absorbed after administration of sodium phosphate solutions, it is estimated that up to 20% of the phosphate dose can be absorbed. When magnesium salts are administered, up to 30% of the magnesium dose of magnesium can be absorbed.
Generally, the onset of action of saline cathartics (characterized by a loose, watery stool) occurs in 3-12 hours after dosing in monogastric animals and within 18 hours in ruminants.
Contraindications/Precautions - Saline cathartics are contraindicated for long-term or chronic use. Sodium containing laxatives are contraindicated in patients with congestive heart failure or congenital megacolon. PEG 3350 solutions are contraindicated in patients with GI obstruction, gastric retention, bowel perforation, toxic colitis or megacolon. Saline cathartics should be used with extreme caution in patients with renal insufficiency, pre-existing water-balance or electrolyte abnormalities, or cardiac disease.
Adverse Effects/Warnings - Except for possible cramping and nausea, adverse effects in otherwise healthy patients generally occur only with the saline cathartics with chronic use or overdoses. Hypermagnesemia manifested by muscle weakness, ECG changes and CNS effects can occur. Hyperphosphatemia with resultant hypocalcemia can occur with chronic overuse or overdoses of phosphate containing products. Hypernatremia can also occur when administering sodium phosphate solutions.
Cats may be particularly sensitive to the electrolyte imbalance effects of sodium phosphate enema solutions and these products are not recommended for use in this species until more data are available.
Overdosage - Symptoms of overdosage of magnesium or phosphate containing laxatives are described above. Treatment should consist of monitoring and correcting any fluid imbalances that occur with parenteral fluids.
If hypermagnesemia occurs, furosemide may be used to enhance the renal excretion of the excess magnesium. Calcium has been suggested to help antagonize the CNS effects of magnesium. Hyperphosphatemia may cause hypocalcemia and parenteral calcium therapy may be necessary.
Drug Interactions - All orally administered saline laxatives may alter the rate and extent of absorption of other drugs by decreasing intestinal transit times. The extent of these effects have not been well characterized for individual drugs, however. Magnesium laxatives should not be administered with tetracycline products
Magnesium sulfate (epsom salt):
a) 0.2 gm/kg diluted in 4 L of warm water administered via nasogastric tube. Administer only to well hydrated animals (ideally in conjunction with IV fluid therapy). Do not treat longer than 3 days or there is an increased risk of enteritis or magnesium toxicity occurring. (Clark and Becht 1987)
b) To reduce absorption of toxicants and GI transit time: 500 gm (as a 20% solution) PO. If mineral oil has been used initially, give saline cathartic 30-45 minutes after mineral oil. (Oehme 1987)
Monitoring Parameters -
1) Fluid and electrolyte status in susceptible patients or if using high doses or chronically.
2) Clinical efficacy
Client Information - Do not give dosages greater than, or for periods of time longer than recommended by veterinarian. Contact veterinarian if patient begins vomiting.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times - Saline cathartic products have apparently not been formally approved for use in domestic animals. They are available without prescription (OTC). PEG 3350 products are available only by prescription and are approved for use in humans.
Veterinary-Approved Products: None located
Saline Laxatives (not an inclusive list):
Magnesium Citrate (Citrate of Magnesia ): powder and oral solution
Magnesium Hydroxide : powder, suspension (Milk of Magnesia )
Magnesium Sulfat e (Epsom Salt ): crystals, powder
Sodium Phosphate, Dibasic or Monobasic: powder
Sodium Phosphate, Dibasi c 900 mg/5ml with Sodium Phosphate, Monobasic 2.4 gm/5ml oral solution
Fleet® Phospho®-Soda (Fleet)
Sodium Phosphate, Dibasic 60 mg/ml with Sodium Phosphate, Monobasic 160 mg/ml rectal solution
Fleet® Enema (Fleet), Fleet® Pediatric Enema (Fleet)
Hyperosmotic Laxatives (not an inclusive list):
Polyethylene Glycol-Electrolyte Solution
OCL® Solution (Abbott) per 100 ml: 146 mg Sodium Chloride, 168 mg Sodium Bicarbonate, 1.29 grams Sodium Sulfate Decahydrate, 75 mg potassium chloride, 6 grams PEG-3350 and 30 ml Polysorbate-80
CoLyte® (R&C) Packets to make 2 liters of solution: 2.92 gm Sodium Chloride, 3.36 gm Sodium Bicarbonate, 11.36 gm Sodium Sulfate, 1.49 gm potassium chloride, 120 gm PEG-3350 (Also available in 1 gallon and 6 liter sizes)
GoLYTELY® (Braintree Labs) contains per 4800 ml container: 5.86 gm Sodium Chloride, 6.74 gm Sodium Bicarbonate, 22.74 gm Sodium Sulfate, 2.97 gm potassium chloride, 236 gm PEG-3350
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