Quick Hit Summary

Tribulus has been hailed as Mother Nature’s (or should I say Father Earth’s?) very own testosterone boosting, muscle popping, libido driving super star supplement. However, an obvious, but often forgotten question, one must remember to ask themselves prior to purchase – “Is there any evidence it actually works?” In Part II of our examination of Tribulus, we examine its effects in humans. One highly questionable study (from a scientific methodology standpoint), indicated that Tribulus supplementation did positively influence various aspects of athletic performance. However, in other more rigorously designed studies, Tribulus had no effect on testosterone production, muscular strength, strength endurance, or body composition changes (ie – % body fat, etc). Unfortunately, the effects of Tribulus on sexual performance/activity have not been directly examined in humans. As discussed in Part I of our look at Tribulus, there is weak to moderate evidence that the supplement did enhance sexual behaviors in rats. That said, based off various lines of evidence discussed within this article, it does not appear that Tribulus lives up to it’s aphrodisiac hype.

Tribulus: Androgen Mania? Increased Athletic Performance? Sex Drive Stimulator?

Figure 1 Tribulus terrestris. Cool looking plant but will it actually increase your testosterone levels, physical performance or sex drive? Image Source³⁷

“Tribulus and Testosterone. Testosterone and Tribulus.” If you repeat that mantra long enough, the words start to sound similar. According to some supplement companies, this is to be expected as Tribulus is Mother Nature’s, (or should I say Father Earth’s?), very own testosterone boosting, muscle popping, libido driving super star supplement … Or is it?

In Part I of this article series we started to unwind this question by first discussing the relationship between testosterone, its fellow androgens and where/how the “mythical” Tribulus fit into the picture. Then, we turned to our furry, fuzzy animal friends and examined how they responded to the supplement. After pouring over the scientific research, we came away with the following conclusion,

Although some research indicates that it does lead to small, acute spikes in androgen levels (ie – testosterone & DHT), just as many show it to have little to no effect on enhancing production of these hormones. With respect to the purported aphrodisiac properties of Tribulus, slight to moderate improvements in sex drive have been noted in rats post supplementation. With respect to its anabolic effects, one study has shown significant weight gain post supplementation.

However, despite some interesting findings in animals, we all know that animal studies do not always play nice and perfectly translate over to humans. (Please refer to Making Sense of Animal Studies for further discussion on this topic.). Thus, in this, the concluding part of our Tribulus “mini” series, I’d like to examine the research in regards to its effect on androgenic hormones production, exercise performance and sex drive in humans.

For those only interested in the potential of Tribulus supplementation to booster your sex life, scroll down to the bottom of the article.

A Quick Review of the Androgenic Pathways and Where/How Tribulus May Fit Into the Picture

A much greater discussion of these relationships can be found in Part I. However, I felt it worthwhile to repost Figure 2 and briefly discuss where/how Tribulus fit into the picture.

Figure 2. The Relationship between Androgenic Hormones. PLEASE NOTE this is a simplified version of the interaction between the various androgenic hormones. Figure created by Sean Casey

Above are the androgenic pathways; As you can see, the end products of these pathways include testosterone, estrogen and DHT. For an in-depth discussion on the endproducts of these androgenic pathways, (which is beyond the scope of this article), I would like to refer you to the following articles by our friends at SuppVersity:

• Testosterone & Muscle Building Part I
• Testosterone & Muscle Building Part II
• Testosterone & Muscle Building Part III
• Estrogen: Friend or Foe?
• Dihydrotestosterone (DHT) - Bigger, Stronger, Faster or just Balder, Fatter and Unhealthier?

How Tribulus may fit into the picture is not 100% for certain. Currently, those in the “pro” tribulus group hypothesize that many of it’s muscle boosting and aphrodisiac properties are attributed to the presence of a steroidal glycosides called protodioscin (PTN) as well as other compounds.³⁶

Does Your Tribulus contain PTN?

Before we get into the research, it’s important to ask a question that most manufacturers hope you never do… Namely, does this herbal supplement actually contain the active ingredients associated with its purported effects??? Different growing conditions, species of the same plant, etc all effect the amount of active ingredient in a given plant.

With that being said, I guess it should not come as a surprise to you that not all Tribulus terrestris contains equal amounts of PTN. In a study completed by Dinchev et al, researchers examined the PTN content of Tribulus terresties obtained from various parts of the world (Bulgaria, Greece, Serbia, Macedonia, Turkey, Georgia, Iran, Vietnam and India).²⁷ PTN was highest in that of European and western Asia. In contrast, it was virtually non-exist in the plants obtained in India & Vietnam. Ganzera et al also found that Tribulus, obtained from southeast Asia (China), has lower PTN content than that found in Bulgaria.²⁸ The PTN content of your supplement can also vary extensively depending on which part of the plant is included in the supplement. Multiple studies have indicated that the highest concentration of PTN is found in the leaves of the Tribulus plant, followed by the fruits and finally the stem.²⁷²⁸. For instance, in terms of dried weight, Dinchev et al found that the PTN content of Tribulus of Bulgarian origin had 10003.5 μg/g in the leaves, 567.95 μg/g in the fruit and 193.3 μg/g in the stem.²⁷

Although I’m not saying if PTN does/does not have beneficial effects (read on to find that out!), this exemplifies A GREAT POINT I WANT TO MAKE ABOUT HERBAL SUPPLEMENTS. Regardless of which kind of herbal supplement you’re buying, always look to see the location the plant is from, the part of the plant actually contained in the supplement and/or some other method (ie-standardization of “X” percent of active ingredient) to ensure what you’re buying actually contains what you think you’re getting!.

Human Based Studies: Athletic Performance & Body Compositional Changes

Okay, now that we’ve discussed the androgenic pathways, how Tribulus may fit into the picture and the affects of tribulus supplementation in animals, we’re back to square one. That is, “Will Tribulus spike your testosterone levels, causing strength levels to shoot through the ceiling?” Since 2000, there have been 3 studies of interest that have examined the effects of Tribulus supplementation on athletic performance in humans.

In a 2000 study completed by Antonio et al., researchers sought to find if Tribulus supplementation would improve one’s mood (perception of vigor & fatigue), upper/lower body strength endurance, and body composition in 15 resistance trained men²¹. The mean ages of the participants were between 20-22 years and they had averaged 3 resistance training sessions per week during the previous year. Additional pertinent info – Although the origin of the supplement was not listed, the supplements were standardized to 45% saponins and protodyosin (PTN).

At the start of the trial, each individual was randomly assigned to either the experimental group, which received a daily Tribulus dose of 7.1 mg/lb (3.2 mg/kg) or the placebo group. For 8 weeks, participants took their respective “pills” and completed a 3 day/week standardized full body resistance training program that followed a traditional periodization pattern (Week 1,2,3: 3×10-12 reps; Week 4,5,6: 3×8-10 reps; Weeks 7, 8: 3×6-8 reps). During this time period, they were instructed to not alter their normal dietary or supplement routine.²¹

Figure 3. Changes in Bench Press and Leg Press Endurance. Data adapted from Antonio et al²¹. LP = Machine Leg Press, BP = Bench Press (barbell). “*” indicates significantly different pre vs. post (p<0.05). Image created by Sean Casey

At the conclusion of the study Antonio et al found no significant differences between and within the groups with respect to improved mood states, overall body weight and % body fat.²¹ For reference, % body fat was assessed via the gold standard technique, underwater weighing. With respect to changes in exercise performance, both groups experienced significant relative gains in lower body endurance as determined by “repping” out on a leg press machine set at 200% body weight (Figure 3). However, only the placebo group experienced significant gains in upper body endurance. As such, the authors concluded that Tribulus had no effect on exercise performance. In regards to their diet, kcal and macronutrient distribution of the participants’ diet did not differ between or within groups throughout the study.

The effects of Tribulus on upper/lower body strength, fat free mass and urinary testosterone/epitestosterone ratio was also examined by Rogerson et al.²² (For reference, the and urinary testosterone/epitestosterone ratio is used to assess exogenous steroid and/or it’s precursor use by the World Anti Doping Association.) In their study, 22 elite Australian rugby players (mean age -19.8 years, mean weight – 194 lbs/88 kg) received either a daily 450 mg dose of Tribulus (standardized at 60% steroidal saponins) or placebo for 5 weeks during the team’s preseason training. In addition to their normal skill sport training, all participants resistance trained 4 days/per week. The sessions focused on total body power and strength. All individuals had a minimum of 12 months resistance training experience and although considerable variation was present, the mean training age for each group was similar (33-36 months). In addition to Tribulus, both groups received two 30 gram packets of protein that were taken daily to ensure a lack of dietary protein did not limit any potential gains in muscle mass. This was the only additional supplement that participants were allowed to take during the time period.

Figure 4. Changes in 2RM Bench Press and seated row. Data adapted from Rogerson et al²². BP = Barbell Bench Press, SR = Machine Seated Row. “*” indicates significantly different pre vs. post (p<0.001). Image created by Sean Casey

Figure 5. Changes in 2RM leg press and deadlift. DL = Machine Dead Lift; LP = Leg Press Machine Data adapted from Rogerson et al²². “*” indicates significantly different from control group. Image created by Sean Casey

After 5 weeks of supplementation, all groups experienced significant gains in 2 rep max (2RM) performance of the following exercises: bench press (free weight), seated row (machine), Leg Press (machine) and Deadlift (machine).²² However, as seen in Figures 4 and Figure 5, *no significant differences or large effect sizes
were noted between groups* (although tribulus did have a moderate effect size on leg press based off the criteria established by Rhea²³). Although both groups increased fat free mass (ie-lean body mass) over the course of the study, no significant differences existed between those receiving the placebo vs. tribulus. The researchers also noted that tribulus did not affect the testosterone/epitestosterone ratio which parallels that seen in a small case study completed by Saudan & Baume²⁴. This lack of change in the test/epi ratio led Saudan & Baume to conclude that Tribulus “… may not be considered a direct precursor of testosterone or as a stimulating agent of endogenous testosterone production …”.

For reference, food intake did not appear to confound results of this study completed by Rogerson et al²². Except for carbohydrates, which was slightly higher in the placebo group (411 vs. 393 g/day), no significant kcal/macronutrient differences were noted between the diets of the athletes when averaged throughout the study.²³

Did you notice the outrageous strength gains over the course of 5 weeks in the study by Rogerson et al?²² If so, you’re not alone as one of the first things I noticed was that in both groups, the bench press shot up 26-33 lbs (12-15 kg), seated row by 35-39 lbs (16-18 kg), deadlift 82-95 lbs (37-43 kg) and leg press by 231-264 lbs (105-120 kg). These strength gains would be impressive even for “newbies” who naturally have great gains when first starting a training program. However, this was in resistance trained athletes. According to Rogerson et al, they all had a minimum of 12 months of lifting experience, particularly in the lifts that were used in the testing of this study. Prior to the start of the preseason, they did have a 6 week off season break in which they “stayed active” but did not have any “formal structured training” during the downtime. Thus, Rogerson et al. hypothesized that the extreme differences pre vs. post testing may have been at least partially attributed to this fact.

The most widely referenced study to promote both the testosterone boosting, athletic enhancing abilities of tribulus is that completed by Milasius et al.²⁵ In their study, 32 athletes, between the ages of 20-22 were separated into 2 separate groups. The control group consisted of 12 individuals (mean weight: 167 lbs/76 kg) who consumed their normal diets. In contrast, the experimental group, which consisted of 20 athletes (mean weight: 165 lbs/75 kg), consumed their normal diet plus received 1875 mg tribulus daily. Neither the origin nor standardization of the formula was discussed by the researchers.

At the conclusion of the 20 day period it was found that tribulus significantly increased anaerobic alactic muscular power (6%), alactate glycolyte working capacity (6%), and VO2 at aneorobic threshold (11%). [25] In addition they found that serum testosterone, as assessed by changes in Km values, increased 13%. For those not familiar with Km values, this has to do with the speed at which “enzymes” carry out a given reaction. In general, when more substrate is available (in this case testosterone), Km values are higher. The control group experienced no significant differences with respect to anaerobic alactic muscular power or alactate glycolyte working capacity. Although the research showing improved athletic performance in athletes post tribulus supplementation is interesting, there are a lot of MAJOR caveats in regards to the research completed by Milasius et al.

• First, the methods section of this article is pretty rough. It does not discuss if/what type of physical training the participants did during the 20 day testing period. If the test subjects simply did “whatever” on their own, those in the Tribulus group may have worked harder/smarter with their training. Thus, leading to the more impressive gains seen in physical performance vs. control group.

• Second, normal dietary practices by the study’s participants were not discussed in the study.

• Third, it appears that the control group did NOT receive any sort of dietary placebo. As you all know after reading The Almighty Placebo, this opens up a whole can of worms! In addition, building off what I mentioned in my first point, if participants were left to do “whatever” on their own, I’m guessing that those in the tribulus group would naturally work harder simply because they would want to take advantage of any benefits tribulus may offer them.

• Fourth, the study’s authors fail to provide reasoning for the unbalanced distribution of participants in each group (control – 12; experimental – 20). Ideally the participants would have evenly been distributed such that 16 were in each group. The fewer people in a group, the more “extreme” results have to be in order to prove significance. Likewise, the more people in a group, the less “extreme” results have to be in order to show statistical significance. Thus, based off how this study is set up, it’s more likely to see statistically significant changes pre vs. post testing in the Tribulus group. In most studies that have uneven groups, the researchers will provide reasoning behind it (ie- people had to drop out because of reasons X, Y, Z).

• Fifth, many of the variables were just assessed in the experimental group (VO2, testosterone, etc). Of course there will be no change in these variables amongst the control group if you don’t measure them!!!

• Finally, this is the first and only study that I’ve come across that reported changes in testosterone levels based off its Km value vs. traditional measurements such as total, free or bioavailable forms. Thus, it’s hard to put into context the importance of this testosterone increase when comparing it to other pieces of scientific literature. In addition, the authors did not elaborate on why the used the Km or how they obtained the Km value. Generally speaking, research articles will discuss the methods (going back to point 1) in which they obtained various lab values.

Human Based Studies: Testosterone Spikes & Sexual Performance

So far we’ve discussed the research in regards to Tribulus and changes in measurements of athletic performance. However, although 2 of the above studies examined the effects of Tribulus on testosterone, neither of them directly assessed total testosterone production (Which has it’s own caveats – Refer to SHBG section of Part I).

Thus, I’d like to shift our focus towards research presented by Neychev & Mitev, who I believe are the only individuals who have examined and published their findings in a peer reviewed journal on testosterone production and Tribulus supplementation. In their study, 21 men, between the ages of 20-36 were randomly assigned to the one of three groups: A control group, a low dose Tribulus group – 22mg/lb (10 mg/kg) or a high dose Tribulus group – 44 mg/lb (20mg/kg). Tribulus was of Bulgarian origin and saponin concentration was 60% of dry weight. Serum androstenedione (Andro) and total testosterone measurements were taken at the following 6 time points: 24 hours pre supplementation (to establish baseline) and 24, 72, 240, 408 and 576 hours into the supplementation protocol. Throughout all points in time, serum Andro and total testosterone levels did not vary within or between groups leading Neychev & Mitev to conclude, “the findings in the current study anticipate that chronic ingestion of either 10 or 20 mg/kg body weight of Tribulus terrestris extract influence neither directly nor indirectly androgen production in young males. “ To the best of my knowledge, there has not been any studies that have looked at the effects of Tribulus supplementation on androgen production in older men.

With respect to the effects of Tribulus on sexual activity and function in humans, I was unable to find any research in peer reviewed journals. Thus, we’re forced to rely on a few pieces of pertinent information. As we saw in Part I, it does appear to enhance sex drive in rats. On the flip side of the equation, there is an abundance of evidence, in both human and animal studies, that androgenic hormone levels are not affected post Tribulus supplementation. Furthermore, the most important (and probably first) question one should ask themselves requires us to take a step away from the whole tribulus question and simply ask:

“Will spiking my testosterone levels even benefit my sex drive?”

Wait a minute… Isn’t testosterone essential for sex drive??? Well, yes, testosterone is essential for sex drive and those suffering from hypogonadism (ie-low testosterone levels) do see improvements in this area upon starting up with testosterone replacement.²⁹³⁰. However, for males with “normal” testosterone levels (total testosterone between 300-1,200 ng/dL)³¹,adding more of the potent hormone to the mix does not appear to enhance sex drive. This was demonstrated in an interestingly designed study completed by Bhasin et al. which involved 61 healthy men, between the ages of 18-35 (mean age: 24-29 depending on group), with mean total testosterone baseline levels ranging between 553-653 ng/dL.³² All individuals were injected with a compound that shut down their body’s ability to release GnRH (Refer to Figure 2), thus limiting their body’s natural testosterone production. Each individual was then assigned to 1 of 5 groups that received a injection of one of the following doses of testosterone enanthate over the course of the following 20 weeks: 25, 50, 125, 300, or 600 mg/week.

Figure 6. Testosterone Levels after 16 weeks of GnRH blocker and various testosterone enthanate doses in younger men. Orange dotted lines represent minimum and maximum for normal testosterone levels in young men (300-1,200 ng/dL)³¹. Serum total testosterone levels were linearly dependent on the testosterone dose (p<0.0001). Data adapted from Bhasin et al.³²

When assessed at week 16, testosterone levels varied between 253-2370 ng/dL depending on the weekly dose one received.³² Worth noting, these measurements were taken 7 days post injection, just prior to their next round of shots. Thus, the serum testosterone levels shown in Figure 6 were, theoretically speaking, at their lowest points during the week. Despite the great range in circulating total testosterone levels, none of the groups experienced any significant changes in sexual activity or sexual desire scores. This held true in both those who saw their circulating total testosterone (and free testosterone – data not shown) drop below or rise above the “normal” circulating levels. This led the research team to later conclude:

“…many aspects of sexual function are normalized at testosterone levels that are at or near the lower limit of the normal range for healthy, young men.”³²

Figure 7. Testosterone Levels after 16 weeks of GnRH blocker and various testosterone enthanate doses in older men. Orange dotted lines represent minimum and maximum for normal testosterone levels in young men (300-1,200 ng/dL)³¹. Please note that these values included data from 8 other men who were part of the “parent study” conducted but did not return surveys on sexual drive/activity. Thus the data from the men in Bhasin et al. was slightly different than that seen here.³³ Data adapted from Bhasin et al.³⁴ Image created by Sean Casey

In a follow-up study, using the same experimental protocol, Bhasin et al, sought to find if testosterone affected sex drive in 44 older men, between the ages of 60-75 (mean age: 64-69 depending on group), similar to that of younger men.³³ Changes in total testosterone are similar to that seen in Figure 7. In contrast to young men³², Bhasin et al found that total testosterone, as well as free testosterone (data not shown), were positively correlated with overall sexual function, waking erections and libido.³³ The greatest difference was noted between those who received those who received 300mg/wk (Total Testosterone levels ~ 1784 ng/dL ) vs. those who received 25 & 50 mg (Total testosterone ~176-274 ng/dL). Why do these differences exist between older and younger men with respect to testosterone and sexual activity? Well, that’s really not fully understood at this point. However, Bhasin et al did hypothesize that it:

“… might be due to the age-related changes in the sensitivity of central as well as peripheral mechanisms that regulate sexual function.”³³; Central being that related to sexual desires, etc, and peripheral related to smooth muscle relaxation, increased bloodflow , etc.

As presented thus far, Tribulus does not lead to stimulate testosterone production. Even if it did, the likelihood of these spikes actually resulting in increased sexual activity or function is likely not going to happen. Therefore, let’s look at the potential of Tribulus acting as a vasodialator. As we all know, blood flow to the reproductive organs is critical for sex. There is some indirect evidence, based off in vitro animal studies that Tribulus may increase relaxation of smooth muscle tissue, potentially via nitric oxide release.³⁵ However, when directly supplied via IV Gauthaman & Adaikan found that tribulus led to an immediate insignificant decrease in blood pressure (2-5 mmHg) and rise in heart rate (18-20 beats/min) that lasted ONLY 2-3 minutes. There have not been any studies in human or animals that have directly assessed changes in blood flow following tribulus supplementation.

In Summary of Tribulus and Sex Drive

There is a lack of evidence regarding tribulus supporting sexual function or activity. It does not lead to spikes in androgenic hormone levels. Even if tribulus did enhance androgen production, outside of men with hypogonadism or older men, it does not appear that increasing circulating testosterone levels would enhance sex drive/function. Furthermore, there is a lack of evidence that it increases blood flow. Taking all this into account, Tribulus once again falls short on the claims made by supplement manufacturers.

Bottom Line

In society today, the urge to get bigger, better, and perform at high levels is the goal of many; regardless of if this is on the athletic field or “between the sheets.” As such, many individuals turn to testosterone boosting neutraceuticals for support. One of the top selling ones on the market today is Tribulus terrestris. Although its hype has reached mythical proportions in some circles (mostly those of manufactures looking to sell you the supplement!), the current scientific evidence falls far short. So for those looking to pack on muscle, lean out and/or simply boost self confidence – clean up your diet (relatively speaking as I don’t encourage anal fear that eating a piece or two of “unhealthy” food will kill you!) & get in the gym… And maybe, just maybe, you will also find a cutie there who will help spark your sex drive as well!

References

²¹ Antonio J, Uelmen J, Rodriguez R, Earnest C. The effects of Tribulus terrestris on body composition and exercise performance in resistance-trained males. Int J Sport Nutr Exerc Metab. 2000 Jun;10(2):208-15.

²² Rogerson S, Riches CJ, Jennings C, Weatherby RP, Meir RA, Marshall-Gradisnik SM.The effect of five weeks of Tribulus terrestris supplementation on muscle strength and body composition during preseason training in elite rugby league players. J Strength Cond Res. 2007 May;21(2):348-53.

²³ Rhea MR. Determining the magnitude of treatment effects in strength training research through the use of the effect size. Journal of Strength and Conditioning Research, 2004, 18(4), 918–920

²⁴ Saudan C, Baume N, Emery C, Strahm E, Saugy M. Short term impact of Tribulus terrestris intake on doping control analysis of endogenous steroids. Forensic Sci Int. 2008 Jun 10;178(1):e7-10. Epub 2008 Feb 20.

²⁵ Milasius K, Dadeliene R, Skernevicius J. The influence of the Tribulus terrestris extract on the parameters of the functional preparedness and athletes’ organism homeostasis. Fiziol Zh. 2009;55(5):89-96.

²⁶ V.K. Neychev , V.I. Mitev The aphrodisiac herb Tribulus terrestris does not influence the androgen production in young men. Journal of Ethnopharmacology 101 (2005) 319–323

²⁷ Dinchev D, Janda B, Evstatieva L, Oleszek W, Aslani MR, Kostova I. Distribution of steroidal saponins in Tribulus terrestris from different geographical regions. Phytochemistry. 2008 Jan;69(1):176-86. Epub 2007 Aug 23.

²⁸ Ganzera, M., Bedir, E., Khan, I.A., 2001. Determination of steroidal saponins in Tribulus terrestris by reversed-phase high-performance liquid chromatography and evaporative light scattering detection. J.Pharm. Sci. 90, 1752–1758.

²⁹ Traish AM. Androgens play a pivotal role in maintaining penile tissue architecture and erection: a review. J Androl. 2009 Jul-Aug;30(4):363-9. Epub 2008 Sep 18.

³⁰ Bhasin S, Basaria S.Diagnosis and treatment of hypogonadism in men. Best Pract Res Clin Endocrinol Metab. 2011 Apr;25(2):251-70.

³¹ NIH. Medline Plus: Trusted Health Information For You. Testosterone. Accessed January 20, 2012 from:http://www.nlm.nih.gov/medlineplus/ency/article/003707.htm

³² Bhasin S, Woodhouse L, Casaburi R, Singh AB, Bhasin D, Berman N, Chen X, Yarasheski KE, Magliano L, Dzekov C, Dzekov J, Bross R, Phillips J, Sinha-Hikim I, Shen R, Storer TW. Testosterone dose-response relationships in healthy young men. Am J Physiol Endocrinol Metab. 2001 Dec;281(6):E1172-81.

³³ Gray PB, Singh AB, Woodhouse LJ, Storer TW, Casaburi R, Dzekov J, Dzekov C, Sinha-Hikim I, Bhasin S. Dose-dependent effects of testosterone on sexual function, mood, and visuospatial cognition in older men. J Clin Endocrinol Metab. 2005 Jul;90(7):3838-46. Epub 2005 Apr 12.

³⁴ Bhasin S, Woodhouse L, Casaburi R, Singh AB, Mac RP, Lee M, Yarasheski KE, Sinha-Hikim I, Dzekov C, Dzekov J, Magliano L, Storer TW. Older men are as responsive as young men to the anabolic effects of graded doses of testosterone on the skeletal muscle. J Clin Endocrinol Metab. 2005 Feb;90(2):678-88. Epub 2004 Nov 23.

³⁵ Adaikan PG, Gauthaman K, Prasad RN, Ng SC. Proerectile pharmacological effects of Tribulus terrestris extract on the rabbit corpus cavernosum. Ann Acad Med Singapore. 2000 Jan;29(1):22-6.

³⁶ Gauthaman, K., Adaikan, P.G.. The hormonal effects of Tribulus terrestris and its role in the managementof male erectile dysfunction – an evaluation using primates, rabbit and rat. Phytomedicine 15 (2008) 44– 54

³⁷ Forest & Kim Starr. Plants of Hawaii. This file is licensed under the Creative Commons Attribution 3.0 Unported license. Image accessed on January 18th, 2012 from: http://commons.wikimedia.org/wiki/File:Starr_030612-0063_Tribulus_terrestris.jpg