Benefits
Female sexual function support (small trials, low certainty)
Multiple trials show modest improvement in libido, sexual desire, and sexual satisfaction in both men and women, including in men with low desire and in women with sexual dysfunction. In the trials in women, serum testosterone rose rather than stayed flat: free and bioavailable testosterone increased in the postmenopausal trial, and the systematic review reported a significant testosterone increase in premenopausal women after three months. So the common claim that the effect is testosterone-independent is not settled, and the nitric oxide route is a proposal rather than a demonstrated mechanism.
Erectile function scores in men diagnosed with erectile dysfunction (mixed trial results)
A meta-analysis of 8 randomized trials in men already diagnosed with erectile dysfunction found tribulus scored better than placebo on the IIEF-5 questionnaire by a mean of 3.23 points, with no difference in total testosterone between groups. The largest single trial (180 men, 12 weeks, 1,500 mg/day) was run with a Bulgarian manufacturer's product and had a company employee among its authors, and an independent 30-man trial of 800 mg/day for 30 days found no advantage over placebo at all. A separate meta-analysis of herbal products for erectile dysfunction judged the tribulus evidence insufficient. Every participant in this literature was a man under medical care for a diagnosed condition. Erectile dysfunction is a medical condition with treatable causes: see a doctor rather than self-treating it.
Urinary tract: traditional use only, and it did not hold up when measured
Sanskrit name 'gokshura' translates to 'cow hoof' (referring to fruit shape that injures cattle hooves) — and traditionally used for urinary stones, dysuria, BPH. When urinary symptoms were actually measured, in the placebo-controlled trial cited on this page, the International Prostate Symptom Score did not change on tribulus (14.4 before, 14.6 after, p = 0.67). A PubMed search for randomized human trials of tribulus alone for kidney stones returned no such trial. Set against that, two published case reports describe acute kidney injury in people taking tribulus, one of them in a man taking it specifically to prevent kidney stones. Do not take tribulus for kidney or urinary support.
Blood pressure and lipids: single small trials, different preparations
Two small randomized placebo-controlled trials have measured these. In 98 women with type 2 diabetes, 1,000 mg/day for 3 months lowered total cholesterol and LDL versus placebo, with no change in triglycerides or HDL. In prehypertensive adults, 6 g/day of powdered tribulus fruit for 2 months lowered blood pressure by 7.7/5.5 mmHg versus 1.9/0.2 mmHg on placebo. Both used preparations and doses unlike a typical standardized capsule, both were single-centre, and neither has been replicated, so this is a starting signal rather than a reason to take tribulus for your heart.
Athletic performance: does not raise testosterone or improve performance
Randomized trials in healthy young men find nothing. In 22 elite rugby league players, 450 mg/day for 5 weeks produced no difference from placebo in strength, in fat free mass, or in the urinary testosterone/epitestosterone ratio; both groups gained strength from the training itself. In 21 young men, 10 to 20 mg/kg/day for 4 weeks left testosterone, androstenedione and luteinizing hormone unchanged versus placebo. A systematic review concluded that tribulus is ineffective for raising testosterone in humans and that the marketing claims are unsubstantiated. This is one of the most over-marketed supplements relative to evidence.
Mechanism of action
Nitric Oxide / cGMP Pathway
Protodioscin increases nitric oxide release from corpus cavernosum endothelium in laboratory preparations. A systematic review proposed this nitric oxide effect, rather than any change in testosterone, as the plausible explanation for what people report. It has not been confirmed as the mechanism in a human study.
Androgen Receptor Sensitivity (not Testosterone)
Increased androgen receptor sensitivity is proposed as a way tribulus could affect libido without changing testosterone. No human study has measured androgen receptor density after tribulus, so this stays a hypothesis, and it does not explain the female trials, where serum testosterone did rise.
DHEA Pathway (Theoretical)
Some studies suggest modest effects on DHEA but not testosterone itself. Mechanism unclear.
Calcium Channel Effects (Cardiovascular)
Steroidal saponins show calcium channel modulating activity in laboratory preparations. Whether this explains the blood pressure fall seen in one small trial of powdered tribulus fruit is untested, so treat the mechanism as unconfirmed.
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.
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.
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. Consistent with multiple other trials in athletes. Established that tribulus does not meaningfully increase testosterone in healthy young men.