Does yohimbe increase penis size?
The answer is yes and no.
Unlike a man’s skull or brain, the penis is an organ with no strictly fixed size. Sometimes it is larger, and sometimes it is smaller.
The functional size of the penis at any given time depends on a multitude of factors: genetics, body chemistry, arousal levels, diet, and environmental temperature.
It also depends heavily on the adrenergic nervous system. Yohimbe contains the active alkaloid yohimbine, which is an alpha-2 adrenergic receptor antagonist. (Yohimbe bark that does not contain sufficient yohimbine is largely a nuisance and is more likely to cause a headache than an erection).
By acting as an alpha-2 adrenergic receptor antagonist, yohimbine can maximize penile engorgement and erection quality, temporarily increasing its functional size during arousal. However, this only lasts as long as yohimbine is active in the body. When the yohimbine is cleared, the penis will retract to its flaccid baseline size. In fact, because alpha-2 blockade increases circulating norepinephrine (which can cause mild vasoconstriction in peripheral tissues), the flaccid penis may temporarily appear slightly smaller than baseline after the medication's erectogenic effects wear off.
This is not what people have in mind when they ask whether yohimbe increases penis size. They want to know whether there is permanent tissue growth.
The medical answer to that is no.