Tongkat ali is the root of Eurycoma longifolia, a shrub of the Malaysian and Indonesian forest floor, used traditionally for energy, libido and fever. The modern product is usually a standardized water extract, several trials use one branded extract, and it is sold almost entirely to men on a testosterone claim.

The human literature is small and mostly from the countries that grow it. A 2022 meta-analysis of five randomized trials found a significant rise in total testosterone; a 2015 meta-analysis of two erectile-function trials found no overall effect. The more interesting recent document is regulatory: in 2021 the European Food Safety Authority looked at the standardized extract as a novel food and concluded its safety had not been established, because the genotoxicity tests came back positive.

The characteristic constituents are quassinoids, eurycomanone above all, with glycosaponins and canthin-6-one alkaloids. The proposed route to higher testosterone runs through the hypothalamic-pituitary-gonadal axis, but a two-week trial in 32 healthy young men found testosterone, free testosterone and estradiol rose while LH and FSH did not, which the authors read as an adrenal rather than gonadal route. A four-week trial in moderately stressed adults reported salivary cortisol down 16% and salivary testosterone up 37%. Which of these is the real mechanism, or whether either is, is not settled.

Testosterone. The 2022 meta-analysis pooled five randomized trials and found a large standardized rise in total testosterone (SMD 1.35) in men, confirmed in the hypogonadal subgroup. Nine studies were reviewed in all. The abstract gives no participant total and no absolute change in testosterone units, and a large standardized effect from a few small trials is the pattern that tends to shrink when bigger trials arrive.

Erectile function. The 2015 meta-analysis found two eligible trials with 139 participants and no significant difference from placebo on the IIEF-5 at 12 weeks (0.91 points, with 89.5% heterogeneity). A subgroup with lower baseline scores improved by 2.15 points.

The best single trial. 45 men around 47 years old with androgen deficiency symptoms were randomized across four arms for six months: 200 mg of extract or placebo, with or without concurrent training. Erectile function improved on both the extract and training, most in the arm that had both. The trial had no arm of extract alone against placebo alone, which its authors list as the main limitation, so the extract's own contribution cannot be separated from exercise.

Young healthy men. Two weeks of 600 mg a day in 32 men raised testosterone, free testosterone and estradiol against placebo. A six-week crossover in 13 male athletes at 400 mg a day found no change in the urinary testosterone to epitestosterone ratio and no change in liver or kidney tests.

Stress. Four weeks of a standardized extract in 63 moderately stressed adults reduced tension, anger and confusion scores and shifted salivary cortisol and testosterone. Mood questionnaires in a small trial.

Three separate signals, none of them from the efficacy trials.

The 2021 EFSA novel-food opinion is the one to read. The standardized water extract, proposed at up to 200 mg a day, was positive in an in vitro chromosome aberration test and then positive in the in vivo comet assay at the highest dose tested (2,000 mg per kg) in stomach and duodenum, which histology attributed to genotoxicity rather than cytotoxicity. The panel concluded the extract has the potential to induce DNA damage and that its safety had not been established under any condition of use.

A 2024 case report describes a 47-year-old man with acute liver injury and jaundice shortly after starting tongkat ali, resolving on withdrawal, the first such report. One case is one case, but herbal liver injury is under-reported rather than over.

Contamination. In a 2006 survey of 100 tongkat ali products on the Malaysian market, 26% contained 0.53 to 2.35 ppm of mercury against a 0.5 ppm limit. That was Malaysia twenty years ago; what is in a given online product now is unknown unless it has been tested.

The six-week athlete trial found no liver or kidney changes at 400 mg a day, and the efficacy trials report no adverse effects, which is what small, short trials usually report.

A dietary supplement in the United States, sold without review of its claims. In the European Union the standardized extract was submitted as a novel food and in 2021 EFSA declined to establish its safety, so it has no cleared status there. A registered traditional medicine in Malaysia, where the mercury survey found a quarter of products out of specification. No regulator anywhere has authorized a testosterone claim.