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Dantrolene is a muscle relaxant used in horses to manage conditions involving excessive skeletal muscle contraction or muscle damage, most notably exertional rhabdomyolysis (tying-up). [1][2] Exertional rhabdomyolysis occurs when muscle cells become damaged during or after exercise, leading to pain, stiffness, and impaired movement.
Unlike centrally acting muscle relaxants, dantrolene works directly on skeletal muscle fibers. It reduces muscle contractility by inhibiting calcium release within muscle cells, thereby limiting the excessive muscle contraction that contributes to muscle injury. This mechanism allows dantrolene to relax skeletal muscle with minimal effects on the lungs or heart. [1]
In horses, dantrolene is commonly used in situations where uncontrolled muscle contraction may worsen tissue damage, such as in horses with a history of exertional rhabdomyolysis or following prolonged recumbency during general anesthesia. [2][3]
Despite its therapeutic benefits, dantrolene must be used with caution. Its use in horses is considered off-label, meaning no equine-specific formulation is approved for this purpose. Dantrolene has also been associated with liver injury (hepatotoxicity), particularly with prolonged administration, and should be used carefully in horses with pre-existing liver disease. [1]
Dantrolene remains a valuable tool for managing muscle hypercontractility in horses when used appropriately. Careful case selection, veterinary-directed dosing, and monitoring are important to maximize benefits while reducing the risk of side effects.
Dantrolene for Horses
Dantrolene is a direct‑acting skeletal muscle relaxant. In human medicine, it is approved to treat conditions such as malignant hyperthermia (a life‑threatening reaction to general anesthesia) and certain types of muscle spasticity. [1]
In horses, dantrolene is used off-label mainly to help manage exertional rhabdomyolysis (tying‑up), a condition in which the horse’s muscles hyper-contract during or after exercise, leading to pain and potential muscle damage. [1][2]
The best‑known trade name is Dantrium®. [1]
Available Forms
Dantrolene is available in oral capsules and as a powder that is mixed with sterile fluid to make an injection. [1]
In horses, it may be given by mouth either as capsules mixed into a small amount of feed or as an oral suspension, or by injection.
Brand and other names for dantrolene include: [1]
- Dantrium®
- Danlene®
- Dantamacrin®
- Dantralen®
- Dantrolen®
- Dantrolene sodium
- F-440
- F-368
- Revonto®
- Ryanodex®
Drug Class: Skeletal Muscle Relaxants
Skeletal muscle relaxants are used to reduce excessive skeletal muscle tightness, spasm or contraction. They may act centrally (on the spinal cord or brain) or directly on muscle.
Centrally acting agents often have sedative effects because they affect the nervous system. [1][3][4] Direct-acting agents such as dantrolene work at the level of the muscle fiber, and typically do not cause drowsiness.
Other skeletal muscle relaxants used in horses include: [1][5]
- Methocarbamol
- Guaifenesin
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Mechanism of Action
Muscle fibers contract when calcium is released from inside the cell. Dantrolene is thought to interfere with calcium release through the RyR1 receptor, making less calcium available to trigger contraction. As a result, the muscle cannot contract as strongly and relaxes more easily. [1][5]
At therapeutic doses, dantrolene does not have major direct effects on the lungs or heart, but some horses may show drowsiness or ataxia (uncoordinated movement). [1]
Food intake can affect how well the drug is absorbed. Long periods without feed before dosing are generally avoided.
The drug is cleared from the body over several hours, with an oral half‑life of roughly three to four hours in horses. [4]
Uses in Horses
The main use of dantrolene in horses is for tying-up (exertional rhabdomyolysis). [1][2][3] Tying‑up refers to a group of muscle disorders in which skeletal muscle becomes painful, stiff, and damaged during or shortly after intense exercise. [6]
Signs of tying-up may include: [6]
- Excessive sweating
- Shortened stride length
- Rapid heart rate (tachycardia)
- Rapid breathing (tachypnea)
- Resistance to forward movement
The use of dantrolene in horses is considered off-label and is prescribed at the discretion of a licensed veterinarian. The manufacturer does not guarantee the safety or efficacy of the drug for this use.
Veterinarians may prescribe dantrolene based on research demonstrating reduced post-exercise muscle enzyme release (such as creatine kinase) and improved control of clinical signs in horses prone to tying-up. [2][3]
Administration Routes
Dantrolene may be given orally as capsules opened and top dressed on feed, or as an oral suspension administered with a syringe. Some veterinarians may administer it intravenously after the powder has been mixed with sterile suspension. [2][4]
Storage
Store dantrolene according to manufacturer instructions. In general, powder is stored at room temperature (25°C or 77°F) in a tightly closed container, protected from light.
Reconstituted injection (powder mixed with sterile fluid) should be used within the time specified in the product leaflet and kept protected from prolonged light; the exact storage temperature varies by brand. [7]
Do not transfer the mixed solution into large glass bottles, as crystals can form (precipitate). [7]
Oral mixtures prepared from capsules may only be stable for a limited time. Always follow the product label and storage instructions for the specific formulation you are using.
Safety Warnings & Precautions
Use dantrolene with extreme caution in horses with known or suspected liver disease, and only under close veterinary supervision. Use with caution in horses with serious heart or lung disease. [1]
Injectable products are only administered by veterinary personnel, as it is important to ensure the product is delivered directly to the vein. Leakage of the alkaline solution into surrounding tissues can cause local damage. [1]
Monitoring
When administered according to instructions from the prescribing veterinarian, dantrolene is usually well-tolerated in adult horses without other pre-existing health conditions.
Horses receiving dantrolene should be monitored for:
- Weakness
- Lethargy
- Gastrointestinal disturbance
- Photosensitivity
- Jaundice (yellow gums, skin, eyes)
Liver enzyme monitoring may be appropriate, especially with long-term use. Contact the prescribing veterinarian if the horse’s condition worsens or new signs appear.
Contraindications
Contraindications refer to circumstances where a drug is to be avoided or only used with caution.
Dantrolene can cause hepatotoxicity (damaged liver cells) and should be used with extreme caution in horses with pre-existing liver disease. It should also be used cautiously in horses with serious heart disease or lung disease, as any additional weakness or systemic effects may be poorly tolerated. [1]
Dantrolene injection has a high pH (9.5 or higher). If it leaks out of the vein into the surrounding tissues (extravasation), it can cause local irritation or tissue damage. Careful intravenous technique and monitoring of the catheter site are important. [1]
Given this, intravenous dantrolene should only be administered to horses by a trained veterinary professional.
Side Effects
Dantrolene works directly on skeletal muscle, so off-target effects in the central and peripheral nervous systems are typically minimal.
Side effects may include: [1]
- Weakness
- Uncoordinated movement (ataxia)
- Lethargy
- Reduced appetite
- Increased urinary frequency
- Gastrointestinal effects
- Hepatotoxicity (liver damage)
If your horse’s condition changes or worsens while on dantrolene, contact the prescribing veterinarian immediately.
Acute Toxicity (Overdose)
Giving too much dantrolene can greatly worsen the typical side effects, leading to profound weakness, marked sedation, stumbling, or severe digestive upset. It may also increase the risk of liver damage. [1]
There is no specific antidote for dantrolene overdose. Treatment is supportive and may include: [1]
- Fluid therapy
- Anti-inflammatory medications
If an overdose is suspected or the horse shows severe or unexpected signs after receiving dantrolene, contact a veterinarian immediately.
Drug Interactions
Formal drug interaction studies for dantrolene in horses are limited. The following drug interactions have been reported in other animals.
Drugs that may increase sedation if used with dantrolene include: [1]
- Benzodiazepines
- CNS Depressants
- Opioids
Use of calcium-channel blockers with dantrolene during malignant hyperthermia is not recommended due to risk of severe hyperkalemia with cardiovascular collapse. [1] Use of estrogens with dantrolene may increase risk of hepatotoxicity. [1]
Dantrolene may increase the effects of vecuronium and other paralytics. [1]
Regulatory Status & Legal Considerations
Dantrolene use in horses is off-label. Regulatory status may vary by jurisdiction; veterinarians prescribe it at their discretion based on supporting research and the individual horse’s condition.
If your veterinarian recommends dantrolene, it is appropriate to discuss potential side effects, expected benefits, duration of therapy, and any competition withdrawal requirements that may apply.
Status in Competition
The FEI Equine Prohibited Substances List lists dantrolene as a Prohibited Substance and Controlled Medication. [8] There is no detection time or withdrawal time listed by the FEI.
As no official detection time is available, the treating veterinarian must set an appropriate withdrawal time based on the individual case, using a safety margin. A detection time (when one exists) is only a guide; the withdrawal time used for competition is often longer to reduce the risk of a positive test.
Always check the current rules for your discipline before starting a new medication for your horse.
Frequently Asked Questions
Here are some frequently asked questions about dantrolene for horses:
Dantrolene is used off-label in horses primarily to manage conditions involving excessive skeletal muscle contraction, such as exertional rhabdomyolysis (tying-up). It may be used before exercise in horses with recurrent tying-up or during treatment of muscle injury. Dosing and use are determined by the treating veterinarian.
Dantrolene is a direct-acting skeletal muscle relaxant. It reduces muscle contraction by interfering with the release of calcium from inside muscle cells. By limiting excessive calcium release, it helps decrease abnormal muscle contraction associated with certain muscle disorders.
Dantrolene may be administered orally as capsules, powder, or suspension, and it can also be given intravenously in a clinical setting after reconstitution. The route, dose, and frequency are determined by the veterinarian based on the horse's condition.
When given before exercise in horses prone to tying-up, dantrolene is typically administered about one hour prior to work. The onset and duration of effect depend on the dose, route of administration, and the individual horse's metabolism.
Potential side effects of dantrolene include muscle weakness, sedation, increased urination, and gastrointestinal upset such as diarrhea or nausea. The most serious risk is liver toxicity, particularly with repeated or prolonged use. Horses receiving dantrolene should be monitored by a veterinarian.
Dantrolene is used off-label in horses, so the drug manufacturers do not guarantee its safety. However, when used under veterinary supervision, dantrolene is generally considered safe for otherwise healthy adult horses. It should be used cautiously in horses with liver disease or other systemic illness. Monitoring may be recommended when the medication is used repeatedly.
Yes. In some horses with recurrent exertional rhabdomyolysis, veterinarians may prescribe dantrolene prior to exercise to help reduce the risk of muscle contraction abnormalities that contribute to tying-up episodes.
Other medications used to manage muscle conditions in horses include methocarbamol and guaifenesin. The appropriate medication depends on the underlying condition, and treatment decisions should be guided by a veterinarian.
Medication rules vary by governing body and discipline. Dantrolene is on the FEI Prohibited Substances list. Always consult current competition regulations and your veterinarian before using it in a competing horse.
Summary
Dantrolene is a direct-acting skeletal muscle relaxant used off-label in horses for conditions such as exertional rhabdomyolysis.
- It works by reducing calcium release from the sarcoplasmic reticulum in skeletal muscle
- Dosing and route are determined by the veterinarian; use in horses is off-label
- The most serious risk is hepatotoxicity; use with extreme caution in hepatic impairment and avoid extravasation with the injectable form
- Side effects include weakness, sedation, GI effects, and increased urinary frequency
- Competition status and withdrawal time should be verified against the current FEI EPSL; withdrawal time is set by the treating veterinarian when no detection time is published
References
- Plumb's Veterinary Drugs. John Wiley & Sons. 2018.
- Edwards. J. G. T. et al. The Efficacy of Dantrolene Sodium in Controlling Exertional Rhabdomyolysis in the Thoroughbred Racehorse. Equine Veterinary Journal. 2003. View Summary
- McKenzie. E. C. et al. Effect of Oral Administration of Dantrolene Sodium on Serum Creatine Kinase Activity after Exercise in Horses with Recurrent Exertional Rhabdomyolysis. American Journal of Veterinary Research. 2004. View Summary
- Court. M. H. et al. Pharmacokinetics of Dantrolene Sodium in Horses. Journal of Veterinary Pharmacology and Therapeutics. 1987. View Summary
- Pang. D. S. J. Anesthetic and Analgesic Adjunctive Drugs. Veterinary Anesthesia and Analgesia. 1st ed. Wiley. 2024.
- Valberg. S. J. Exertional Myopathies in Horses. Merck Veterinary Manual. 2024.
- Dantrolene sodium for injection. FDA. 2024.
- Equine Prohibited Substances List. FEI. 2024.
