Reserpine is a naturally occurring substance used for centuries in herbal medicine in India.
It is extracted from the root of Rauwolfia serpentina or Rauwolfia vomitoria plants.
In horses, is it predominantly used as a long-term sedative.
Horse owners are more likely to know it as Rakelin, an injectable medication.
The active ingredient, the alkaloid known as reserpine, was isolated in the 1950s and became a treatment for blood pressure in humans.
In herbal medicine, the root of the plant was brewed into a tea for use in lowering blood pressure.
Its sedative properties in horses result from the agent irreversibly binding to the storage vesicles of neurotransmitters.
It is typically used where a prolonged calming effect is required, perhaps in a horse that is proving overly anxious or difficult to handle.
It is sometimes used as a calming agent to help in the training of stressed young horses.
It can take days to reach full effect and will have subtle sedating effects for many days after the last injection.
Testing for reserpine in horses has improved greatly in recent years, but there are said to be related herbs and plants in supplements, pastures and even hay which can also trigger positive drug tests.
Reserpine is said to have a long and variable withdrawal period.
The drug can have undesired side-effect in horses, from diarrhoea and sweating to increased risk of gastric ulcers and depression.
It can cause male horses to drop their penises, with penile paralysis is a risk in stallions.
Rakelin is given by injection, usually weekly during a long-term course.