Reserpine is an alkaloid chemical prepared from the plant Rauwolfia serpentian. The drug has been used to produce a prolonged calm or sedated state in horses. There is a very small therapeutic window with toxicity occurring at less than 25% more than some of the therapeutic dose recommendations. This report consists of a description of its pharmacology, toxicity, dosages, and detection times. Links to several clinical reports are provided or reserpine (Serpasil®) in horses and equine. Most information on pharmacology are the results of work in other species as none has been conducted in horses. Not included in this article but at the bottom of the page are discussions on reserpine use in the horse.

Reserpine is readily absorbed from either intramuscular or oral administration. It is distributed to a wide variety of tissues including the brain. Small amounts are excreted in the urine with the majority excreted by the liver into the bowel.

Reserpine is a sympatholytic drug that causes the post ganglionic adrenergic neurons to deplete themselves of catecholamines (norepinephrine). The transient deficiency of norepinephrine disables the sympathetic branch of the autonomic system resulting is a profound parasympathetic tone. Remember the parasympathetic system is the "rest and digest" part of the autonomic system. There also is also some depletion of serotonin and dopamine in the central nervous system.

This binding to the neurons is reversible but a prolonged action occurs due to the high lipid solubility of the drug into body tissues and slow release back into the blood. There may also be an as yet undocumented entero-hepatic circulation.