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Tribulus terrestris L., a member of the Zygophyllaceae family, is an annual herb found in many areas of the world, including the US and Mexico, the Mediterranian region, and throughout Asia [1-4]. It is also used in the folk medicine of many areas, such as India, China, and Turkey [1, 3]. Traditional uses include treatment of sexual impotency, edema, abdominal distention, cardiovascular disease, kidney problems, and as a cough remedy [1, 3]. It also has a reputation for having anabolic effects in some areas of the world [4]. A large amount of potential active components have been identified in tribulus, including steroidal saponins, lignanamides, alkaloids, and flavanoids [5]. The amount of these components can vary significantly based on the region of origin and the part of the plant used [1-2]. HPLC analysis of different tribulus supplements revealed significant product-to-product variations in active ingredients [4], presumably due to different regions of origin. The current research indicates that the steroidal saponins, particularly the dominant saponin protodioscin, are responsible for the pharmacological activities of tribulus [4].

One of the most well-known properties of tribulus is as an aphrodisiac, and as a putative testosterone elevator. Tribulus does increase sexual function in animal models, and also reportedly improves libido in humans [6-7]. However, this is not necessarily indicative of a testosterone-increasing effect – there are other possible reasons, such as the hypotensive effect of tribulus. There is limited information regarding the effect tribulus ultimately has on testosterone levels. Translated Bulgarian research indicates that tribulus increased testosterone and luteinizing hormone (LH) levels in depressed men who were part of infertile couples [8]. When protodioscin is administered to castrated rats, it increases levels of testosterone, LH, DHEA, and DHT [7].

Two mechanisms have been proposed for these increases – the first being that protodioscin directly increases LH, and the second being that protodioscin increases levels of DHEA (which would ultimately mildly increase testosterone), perhaps by functioning as a precursor [3, 9]. The neural effects of DHEA would also explain the aphrodisiac properties [7]. However, it should be noted that if the latter is the case, it would open the possibility that tribulus could ultimately lead to lower natural production of testosterone. The ultimate effect of tribulus on testosterone levels in healthy humans, and consequently the effect on body composition and exercise performance, remains to be seen. The present research is not promising – a trial in fifteen resistance trained subjects found that tribulus did not improve body composition or strength over an eight week period [10]. On the other hand, this trial may not have even used a tribulus extract with adequate quantities of the active ingredients.

There has also been clinical research on the use of tribulus in many medical conditions, usually with positive results. Biological properties of tribulus extracts include diuretic properties, increased endothelial nitric oxide, direct smooth muscle relaxant effects, and ACE inhibition [2, 11]. In vitro, tribulus also inhibits COX-2 [12]. Cardiovascular conditions in which tribulus has showed promise include high blood pressure, high cholesterol, coronary heart disease, and angina pectoris [2-3, 13]. In both normal and diabetic mice, tribulus decreases serum glucose, perhaps by inhibiting gluconeogenesis [14-15]. Tribulus also inhibits stone formation in experimental animals [3] and may have hepatoprotective properties [16]. It may also be useful in the treatment of vitiligo and bacterial infections [3]. A number of other uses have been reported in traditional medical systems.

In human trials using tribulus, it has been reported as safe and side effect free [2-3, 17]. One trial reported no negative effect on the circulatory system or hepatic and renal function [17]. Toxic effects have been observed in sheep fed large amounts of tribulus [18], but are not likely to be applicable to normal supplemental use by humans. The LD50 of tribulus-derived saponins in mice is 813 mg/kg, which is considerably higher than the commonly used dose (which rarely exceeds more than a gram daily of saponins). In conclusion, tribulus is a relatively safe supplement that has aphrodisiac properties and may be beneficial to the cardiovascular system. The effect on testosterone levels and related variables is not well established.