As a result of individual temperament or being forced to engage in activities in which they wouldn’t normally engage in the wild (e.g., carrying a rider, walking into dark stables, traveling in a trailer, riding along busy roads) some horses develop excitability. In extreme cases, a horse may be so nervous or excitable that nearly every situation is a challenge.

Pain can be associated with poor behavior in excitable or difficult horses; thus, it is advisable to have a veterinarian assess any horse before making any changes or considering medications to address the issue. In some cases, excitability can be modified through training or diet.

Reserpine for Excitability in the Horse

Reserpine is an alkaloid derived from the plant Rauwolfia serpentina, and is used as a long-acting tranquilizer for horses. It has also been useful in managing horses on forced stall rest.² Reserpine method of action is via blocking the storage of some of the brain's chemical messengers, including a neurotransmitter called norepinephrine.¹ Reserpine usually takes hours to fully take effect and has subtle sedating effects long after the last dose is given. 

Reserpine had historically been used as a treatment for agalactia in mares with fescue toxicosis; however, this use has mainly been replaced by other drugs. Reserpine irreversibly blocks transport of norepinephrine, serotonin, and dopamine into storage vesicles. Without storage, the neurotransmitters are metabolized in the cytoplasm of presynaptic nerve terminals by monoamine oxidase and catechol-O-methyltransferase.² Reserpine is highly lipid-soluble and can penetrate the blood–brain barrier, which results in prolonged duration of action. 

Reserpine is contraindicated in patients hypersensitive to reserpine or components of reserpine formulations. It should be used with extreme caution in horses with low blood pressure or low heart rate. Reserpine is considered a prohibited substance in most sanctioned competitions and can be detected through high-performance liquid chromatography.²

Adverse effects of reserpine in horses are associated with decreased sympathetic tone and adrenergic inhibition. Common adverse effects reported in horses include diarrhea with or without signs of abdominal pain or colic (eg, pawing, getting up and down, pacing, restlessness, rolling, kicking or biting at side, stretching out/straining, shifting weight), muscle tremors/fasciculations, bradycardia, and hypotension.³

Dosing recommendations for reserpine in horses are as follows (Adapted from Plumb’s Veterinary Drugs):

As a tranquilizer (extra-label): Since individual horses can differ greatly in their sensitivity to reserpine, dosages should be adjusted to the individual horse based on response. Commonly used dosages for oral administration are 0.002 – 0.008 mg/kg PO once daily (1 – 4 mg/500 kg horse PO once daily). A similar IM dose is recommended; however, administration is once weekly, or even less often, based on response. The maximum tolerated dose of reserpine in horses is 10 mg, although doses as low as 5 mg may cause adverse effects.

For fescue toxicosis (extra-label): 0.01 mg/kg by mouth every 24 hours.

Where to buy Reserpine

Reserpine is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. RESERPINE 1 MG/SCOOP by NexGen Pharmaceuticals is indicated for the mitigation of chronic excitability in the horse.

This product carries numerous potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹equimed.com.

²Plumb’s Veterinary Drugs.

³Tobin T, et. al. Drugs and the performance horse. Springfield, IL: Charles C Thomas; 1981:245-260.