Eurycoma longifolia — commonly known as Tongkat Ali, or Long Jack — has been used in Southeast Asian traditional medicine for centuries. In the past two decades, it has become one of the most studied natural compounds in men’s health research, with a growing body of human clinical trials examining its effects on testosterone levels, vitality, stress, and body composition.
So what does the science actually say? The honest answer, grounded in the current evidence base, is nuanced: Tongkat Ali shows genuine and meaningful effects in specific populations, appears to work through identifiable biological mechanisms, and varies considerably in potency depending on how it’s extracted and standardized. Here is a plain-language review of where the evidence stands.
The Biological Mechanisms
Tongkat Ali root contains more than 65 identified bioactive compounds, including quassinoids (of which eurycomanone is the primary active compound), glycosaponins, eurypeptides, and alkaloids. The mechanisms proposed for its testosterone-supporting effects include:
- Aromatase inhibition: Eurycomanone has demonstrated the ability to inhibit the aromatase enzyme, which converts testosterone into estrogen. By reducing this conversion, more testosterone remains in active, bioavailable form.
- CYP17 enzyme activation: Eurypeptides are believed to increase the activity of CYP17 (17α-hydroxylase/17,20-lyase) in the testes, an enzyme involved in converting precursor hormones into androgens including DHEA and testosterone.
- HPG axis modulation: Tongkat Ali appears to influence the hypothalamic-pituitary-gonadal axis, the central hormonal communication pathway governing testosterone production.
- Cortisol reduction: Multiple human trials have documented reductions in cortisol following Tongkat Ali supplementation. Since cortisol and testosterone are biological competitors — elevated cortisol suppresses testosterone production — this adaptogenic effect has downstream hormonal significance.
What the Human Clinical Trials Show
A 2022 systematic review and meta-analysis of randomized controlled trials (RCTs) found that nine published RCTs on Tongkat Ali showed significant increases in total testosterone following supplementation. The magnitude of increase varied considerably across studies — ranging from approximately 5% to over 60% — depending on the population studied, the extract used, and the dose administered.
One of the more rigorous trials, published in Food & Nutrition Research, enrolled 105 older men (aged 50–70) with documented low testosterone (below 300 ng/dL) and randomized them to 100mg of Eurycoma longifolia extract, 200mg, or placebo for twelve weeks. Both 100mg and 200mg doses increased total testosterone compared to placebo. The 200mg dose — but not the 100mg — also produced significant improvements in muscle strength. Participants also reported improvements in perceived symptoms of aging and fatigue.
A pilot study in physically active male and female seniors (aged 57–72) supplemented with 400mg of Tongkat Ali extract daily for five weeks found significant increases in total and free testosterone concentrations, as well as improvements in muscular force, compared to baseline.
A 2024 study published in the Tropical Journal of Natural Product Research compared three Tongkat Ali species and found that Yellow Tongkat Ali (Eurycoma longifolia) produced the largest effect on testosterone secretion, increasing it by approximately 41% in the study model.
Where the Evidence Is More Cautious
Not all studies are positive, and intellectual honesty requires acknowledging the full picture.
A 2024 study in applied sciences enrolled exercise-trained males and females supplemented with 400mg of Tongkat Ali daily for four weeks. In this population — younger, physically fit, with normal baseline testosterone — Tongkat Ali did not affect body composition, free testosterone, or cortisol. The authors concluded that if Tongkat Ali produces meaningful effects, they are most likely to occur in older hypogonadal males rather than in younger trained individuals with already-healthy testosterone levels.
The NIH’s LiverTox database notes that human studies have generally shown mild increases in serum total testosterone, often requiring several weeks for a full response, with variable effects on free testosterone and sex hormone binding globulin depending on the study. It also flags that reports of liver injury attributed to Tongkat Ali in bodybuilders have appeared in the literature — though these cases are primarily associated with unregulated or potentially contaminated products rather than standardized clinical extracts.
The Standardization Problem: Why Most Products Don't Work
Perhaps the most important practical finding in the Tongkat Ali evidence base is this: most products on the market contain virtually no meaningful amount of the active compound.
Independent laboratory analysis conducted in 2025 tested popular retail Tongkat Ali products for actual eurycomanone content. The results were stark: potency varied by more than 100-fold between products. The highest-quality product contained 12mg of eurycomanone per dose. Several products contained less than 0.5mg. Multiple products showed no detectable eurycomanone at all, despite labeling and pricing suggesting otherwise.
This explains the widespread consumer experience of “Tongkat Ali doesn’t work” — in many cases, the product being consumed contains no meaningful amount of the compound the clinical trials actually studied.
Clinical doses in the human research typically use standardized root extracts at 100–400mg per day, with eurycomanone standardization typically at 1.5–2% or higher. Products that do not disclose their standardization percentage, or that rely on generic “ratio extracts” without verifying bioactive content, are unlikely to replicate trial results.
Aleaf Labs Testosterone Booster contains Tongkat Ali standardized to 1.5% eurycomanone, at a clinically informed dose, formulated to Health Canada NHP standards (NPN 80088086).
The Bottom Line
Tongkat Ali is among the best-supported natural compounds for testosterone-related outcomes in the current evidence base. The effects are meaningful and replicable in the population most likely to need support — older men with low or declining testosterone — and the mechanisms are increasingly understood. It is not a replacement for testosterone therapy in cases of clinical hypogonadism, and it is not equally effective in younger men with healthy baseline hormone levels.
What it is: a botanically derived, clinically studied, mechanism-supported natural compound with a legitimate place in a men’s health formulation — provided the product actually contains what it claims.
References
- Leisegang K, et al. Systematic review and meta-analysis of Tongkat Ali RCTs. 2022.
- Chinnappan SM, et al. Effect of Eurycoma longifolia (Tongkat Ali) on testosterone in older men with low testosterone. Food & Nutrition Research. 2021.
- Henkel RR, et al. Tongkat Ali as a potential herbal supplement for physically active male and female seniors. Phytotherapy Research. 2014.
- Tropical Journal of Natural Product Research. Comparison of testosterone effects across Tongkat Ali species. 2024.
- Smith SJ, et al. The effect of Tongkat Ali supplementation on body composition in exercise-trained males and females. Applied Sciences. 2024. MDPI.
- NIH LiverTox. Tongkat Ali. October 2024. ncbi.nlm.nih.gov/books/NBK609015
- PricePlow Blog. Independent lab analysis of Tongkat Ali eurycomanone content. June 2025.
- Examine.com. Research Breakdown on Tongkat Ali. September 2025.
- WHYZ. Tongkat Ali: Benefits, Dosage & Safety. March 2026.