Benefits
Sexual function in women
A review of 5 small randomized trials in 279 premenopausal and postmenopausal women with sexual dysfunction found higher sexual function scores after 1 to 3 months on tribulus, but rated the certainty of the evidence as very low. Testosterone did not stay flat: it rose in the postmenopausal trial (free and bioavailable) and in premenopausal women at three months, so the effect is not shown to be testosterone-independent.
Erectile function in men
A meta-analysis of 8 randomized trials in men diagnosed with erectile dysfunction found tribulus beat placebo on the IIEF-5 by a mean of 3.23 points, with no difference in testosterone. The largest trial (180 men) had a manufacturer employee among its authors, a 30-man trial of 800 mg/day found no benefit, and two other reviews judged the evidence low or insufficient, so the effect is uncertain.
Urinary symptoms in men: unchanged in a placebo-controlled trial
Tribulus is sometimes taken for urinary complaints, including to prevent kidney stones. In a 3-month placebo-controlled trial in 70 men with low testosterone and urinary symptoms, the International Prostate Symptom Score did not change on tribulus (14.4 before, 14.6 after, p = 0.67). Two case reports describe acute kidney injury in people taking it, one to prevent stones, so it is a poor choice for kidney or urinary support.
Blood pressure and cholesterol
In 98 women with type 2 diabetes, 1,000 mg/day for 3 months lowered total and LDL cholesterol versus placebo. In prehypertensive adults, 6 g/day of powdered fruit for 2 months lowered blood pressure by 7.7/5.5 mmHg versus 1.9/0.2 mmHg on placebo. A 180-man trial of a standardized extract found no difference in cholesterol or blood pressure, so the results are inconsistent.
Strength and testosterone in young men: unchanged versus placebo
In 22 elite rugby league players, 450 mg/day for 5 weeks gave no more strength or fat free mass than placebo, and the urinary testosterone/epitestosterone ratio did not change. In 21 young men, 10 to 20 mg/kg/day for 4 weeks left testosterone, androstenedione and luteinizing hormone unchanged. A review of the published studies called the testosterone marketing claims unsubstantiated.
Mechanism of action
Nitric oxide (proposed)
A systematic review proposed nitric oxide release, rather than any change in testosterone, as the most plausible explanation for the effects people report. It has not been confirmed as the mechanism in a human study.
Hormone effects in animals, not in people
In primates given tribulus extract intravenously, testosterone, dihydrotestosterone and DHEA sulphate rose. In healthy young men, 4 weeks of tribulus left testosterone, androstenedione and luteinizing hormone unchanged, and in a two-day test in two women, urinary DHEA did not change.
Clinical trials
Randomized, double-blind, placebo-controlled trial of tribulus 750 mg/day for 120 days in 45 healthy sexually active postmenopausal women reporting diminished libido; 36 completed, with 3 in each arm withdrawn for side effects (de Souza et al. 2016, Menopause).
45 postmenopausal women randomized, 36 completed.
On the Sexual Quotient questionnaire the tribulus group improved in desire, arousal and lubrication, pain and anorgasmia while the placebo group did not. On the Female Sexual Function Index, however, both groups improved in nearly every domain, with only lubrication improving in the tribulus group alone, so the placebo response was substantial. Free and bioavailable testosterone rose in the tribulus group. A later systematic review graded this body of evidence as very low certainty.
Double-blind, match-paired randomized trial of a tribulus extract at 450 mg/day versus placebo for 5 weeks of preseason heavy resistance training in 22 elite male rugby league players, 11 per group (Rogerson et al. 2007, J Strength Cond Res).
22 elite male Australian rugby league players, mean age 19.8 years.
Strength and fat free mass increased significantly in both groups from the training itself, with no difference between tribulus and placebo, and the urinary testosterone/epitestosterone ratio was likewise unchanged. The urinary ratio, checked against the anti-doping limit, was the only hormone outcome reported, so this trial alone does not show whether serum testosterone changed.