Yohimbe is a phytopharmaceutical ("herb") originating in western Africa. It consists of the bark of Pausinystalia yohimbe (syn. Corynanthe yohimbe), an evergreen tree. While the bark is rich in alkaloids (up to six percent by weight), the accepted active constituent is yohimbine, which makes up between one half and one percent of the total weight of the dry yohimbe bark.
Pharmacologically, yohimbine is a stimulant (technically, it's a highly-selective alpha-2 adrenergic agonist). However, it's most commonly-attributed use — stemming from its traditional African use — is as a male sexual stimulant, especially in the treatment of erectile dysfunction. Any observed sexual effects are likely due simply to the stimulant activity of the alkaloid in combination with user expectations.
Yohimbine is used in veterinary medicine as a mild stimulant capable of reversing xylazine anesthesia in both large and small mammals.
Another popular use of yohimbine is common with many other natural and artificial stimulants: weight loss. Being a stimulant akin in action to caffeine, some weight loss may be observed simply due to the heightened state of activity a yohimbine user remains in. Various oral and topical formulations containing some quantity of yohimbine are peddled as solutions for body fat reduction. Creams and patches are billed as useful for "sculpting", while capsules are sold as weight-loss supplements. This in combination with yohimbe's other reputation as an enhancer of the male libido, it has become popular in the bodybuilding community.
As any stimulant, care must be taken with its use. It has many of the side effects of ephedrine (and the natural source of ephedrine, ephedra), such as high heart rate, heart palpitations, hypertension, anxiety, and with prolonged use, chronic overstimulation leading to insomnia.
Yohimbine also has unique side effects which must be taken into account when using it. The primary side effects of large (greater than fifty milligrams per day) doses of yohimbine are neurological: monoamine oxidase inhibition and serotonin and dopamine receptor desensitization.
Monoamine oxidase is an enzyme necessary for the metabolism of excess neurotransmitters, as well as certain dietary components such as the amino acid tryptophan. Serotonin and dopamine are the two neurotransmitters suspected responsible (in too low of quantity) for clinical depression. Ordinarily, monoamine oxidase inhibitors (MAOIs) relieve depression by slowing the metabolism of serotonin and dopamine. However, in combination with desensitization of the receptors for these neurotransmitters, MAOIs can both exacerbate depression and interfere with other treatments.
Many drugs are dependent on the body's monoamine oxidase to remove them from the bloodstream, and dosages are calculated on this assumption. Any drug which interacts poorly with MAOI antidepressants should not be taken in concert with yohimbine-containing products.
Yohimbine is also diuretic, as are caffeine and many other adrenergic stimulants. This can exacerbate problems having to do with blood pressure and kidneys function.