Tribulus (Tribulus Terrestris) is an herb that has been used in China and India for centuries that is has been often used to treat low sex-drive, erectile dysfunction, and infertility.
Recently, it has become a popular supplement to enhance athletic performance. The theory behind this benefit comes from research done in Russia and Bulgaria that indicates that Tribulus may increase testosterone levels by increasing Luteinizing Hormone (LH). LH is a hormone produced by the pituitary gland and plays a role in regulating natural testosterone production, which can lead to increases in size and strength.
It should be noted however, that there are conflicting studies on this supplement that indicate that it has no effect on testosterone, androstenedione, or LH compared to those who have not taken Tribulus.
As always, it is recommended to consult a physician before taking any supplements, especially those that have conflicting studies.