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sa-LEE-nee-um - Pronunciation guide

Brand Names

  • E-Se
  • Equ-SeE
  • Selenium
  • Vitalize Equine Pasture Mate


Selenium, in conjunction with vitamin E, is necessary for proper immune system function and to protect the integrity of cell membranes. However, a delicate balance exists between too little selenium and too much.

In many areas of the country, horses ingest sufficient selenium while foraging, but in other areas, selenium needs to be part of the horse's diet. Chronically selenium-deficient horses will often "tie-up" because of a degenerative condition of the muscles, known as rhabdomyolysis. Severe muscle cramps result in stiffness, sweating, and increased pulse. Broken-down cells can cause the horse's urine to appear brown.

Many different sources of selenium are available for horses. A free-choice salt/mineral mix containing selenium is the safest form of supplementation. When horses live in a selenium-deficient area, one consistent source should be used, and care should be taken to make sure the horse doesn't ingest several products containing selenium because of the danger of selenium toxicity.

In severe cases of selenium deficiency, an injectable product may be used to increase selenium levels immediately.


Selenium is used to supplement the horse's diet to prevent muscle cramps and breakdown of muscle cells that cause "tying-up." Adequate selenium in the diet is also necessary for a healthy heart and respiratory system.

Lack of selenium in the diet may impair reproductive performance and decrease resistance to disease. In cases of severe selenium deficiency, an injectable product may be used for immediate relief, with oral supplementation used thereafter.

Selenium is also combined with other vitamins, minerals, and nutrients as a supplement.

Dosage and Administration

Method Dosage
(click row for calculator)
Concentration Period Duration
Oral 0.3 mg/kg 1   Daily NA
Intramuscular injection 2.5 mg/100lb 2.5 mg/ml Daily NA


  • 1Consider selenium concentrations in the total daily feed intake. The total added selenium should not exceed 0.3 mg/kg (0.3 ppm) in the ration or 3.0 mg selenium per horse per day.
  • Because of potential toxicity, selenium levels should be measured prior to supplementing feed with selenium or giving selenium injections.
  • Extra-label use of drugs in treating animals is allowable only by licensed veterinarians within the context of a valid veterinarian-client-patient relationship, and does not include drug use in treating animals by the layman (except under the supervision of a licensed veterinarian).
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your equine appears to feel better, the entire treatment plan should be completed to prevent relapse.
  • This medication may be available in forms and concentrations not noted in the above table. Always check the label and literature provided with the medication about the form and concentration and DO NOT USE the calculator if the information differs.
  • Calculator is for educational purposes only. Follow your veterinarian's instructions regarding use of this, or any medication.

Side Effects

Excessive intake of selenium will result in chronic selenium toxicity, also called "alkali disease."


Cases of toxicity, due to selenium in water, excessive use of supplements, or environmental contamination have been reported. Follow the advice of a veterinarian who knows the area where horses forage and can determine whether or not a selenium supplement is needed to prevent danger of toxicity.

In some cases, a veterinarian may take a blood sample and have it analyzed for concentrations of selenium or gluathione peroxidase, an enzyme that requires selenium.


None reported.


Overdose is characterized by watery diarrhea, high temperature, weak and rapid pulse, labored respiration, blue mucous membranes, and dilated pupils. Selenium toxicity can lead to death, so the services of a veterinarian should be sought at the first indication of toxicity.


Equimines Selenium Liquid Solution with Vitamins C and E, GinsinEquimines Selenium Liquid Solution with Vitamins C and E, Ginsin

Finish Line Selenium Powder with Vitamin  E  Finish Line Selenium Powder with Vitamin E


About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.