Tribulus terrestris is an herbal supplement mainly used by men for increasing sex drive and building muscle mass. Although tribulus is promoted for producing large gains in lean muscle mass and strength, research published in the May 2007 issue of the Journal of Strength and Conditioning Research did not support this claim. Some preparations are sold as Bulgarian Tribulus terrestris, as tribulus often is harvested from Bulgaria, has a history of usage in eastern Europe, and is commonly associated with Bulgarian strength athletes. Physician and medical writer Ray Sahelian advises that no significant tribulus side effects have been reported, but cautions that research is lacking about the effects of long-term usage.

Metabolic and Cardiovascular Effects

Sahelian notes anecdotal feedback from patients about Tribulus terrestris side effects. Reported effects include feelings of warmth, slightly faster heart rate, increased energy and restlessness. The side effects are more common and pronounced with doses above 500 mg. The Physicians' Desktop Reference lists slightly low blood pressure as a possible side effect of tribulus. Low blood pressure can cause lightheadedness, particularly when getting up from sitting or lying down.

Photosensitivity

Bulgarian Tribulus terrestris can cause skin to become more sensitive to the sun, a condition known as photosensitivity. This increases the risk of severe sunburn. The Physicians' Desktop Reference cautions people using this herbal supplement to apply sunscreen before exposing skin to sunlight.

Hormonal Considerations

Bulgarian Tribulus terrestris may raise levels of testosterone and other male hormones, so men with prostate issues may not be able to safely use this supplement. EMedTV notes that Tribulus terrestris could theoretically worsen an enlarge prostate, elevate the risk of prostate cancer or worsen prostate cancer. Talk with your health care provider before using tribulus if you have either of these conditions or are at increased risk for either condition. Additionally, the possible hormonal effects of tribulus are of concern for women who are pregnant or may become pregnant. If tribulus increases levels of male hormones, the supplement could cause masculinization of genitals in a female fetus. Research with animals also indicates that tribulus might have negative effects on fetal brain development, according to eMedTV.

Allergic Reaction

As with any herbal supplement, some people may experience an allergic reaction to Bulgarian Tribulus terrestris. Signs as listed by the Physicians' Desktop Reference may include trouble breathing, tightness in the chest or throat, chest pain, itchy skin, unexplained swelling, hives or a rash. An allergic reaction to tribulus should be considered a medical emergency because it could lead to life-threatening anaphylaxis, a shock reaction that causes a sudden drop in blood pressure and severe difficulty breathing.