Reserpine is a naturally occurring drug that has been used for centuries in ancient India. It is extracted from the root of Rauwolfia serpentina or Rauwolfia vomitoria, plants found in India and Africa. In traditional herbal medicine, the root was brewed as a tea and used in humans to treat hypertension, insanity, snakebite, and cholera. The purified alkaloid, reserpine, was isolated in 1952 and is considered the first modern drug for the treatment of hypertension. Reserpine irreversibly binds to the storage vesicles of neurotransmitters, particularly norepinephrine, serotonin and dopamine. Eventually, catecholamine depletion occurs because of the body's inability to store these neurotransmitters. It is an unusual drug; it takes many hours or days to reach full effect and continues to have some subtle sedating effects for many days after the last dose.

Horses

Reserpine is used as a long acting tranquilizer in horses. It is used to sedate excitable or difficult horses that are on enforced rest. It is sometimes used illicitly for the sedation of show horses, sale horses or in other circumstances where a "quieter" horse might be desired. Until relatively recently reserpine was difficult to test for, but there are now sensitive and accurate tests. Blood testing for reserpine use can be complicated by related herbs and plants found in supplements, pastures and hay which can also cause a positive drug test. Reserpine was once used in pregnant mares in an attempt to treat "fescue toxicosis." Domperidone has largely replaced reserpine for this use.

Side Effects

• Different horses vary greatly in their sensitivity to this drug.

• Common side effects include, colic, gastrointestinal upset, mild diarrhea that may last for days, and sweating over the back and hind legs. Signs of sedation include depression, droopy eyes, and a dropped penis.

• Reserpine increases gastric secretion in humans and increases the risk of ulcers.

Precautions

• There is little published information on the clinical use of reserpine in horses. As a consequence, much of the available information is anecdotal and should be considered as such.

• Reserpine causes male horses to drop their penis; penile paralysis in stallions is a possible side effect.

• Reserpine is a prohibited substance in most sanctioned competition and is a frequent cause of drug violations due to the long and variable withdrawal period. Some herbal products have been implicated in positive tests for reserpine.

Drug Interactions

• Reserpine may interact with drugs used for general anesthesia.

• Methamphetamine is an antidote to reserpine.

Overdose

Overdose of reserpine increases the risk and the severity of the above-mentioned side effects.