Eurycoma longifolia Jack is a plant also known as Longjack, Malaysian Ginsengor Tongkat Ali that has gained notoriety as a symbol of man’s ego and strength by the Malaysian men because it increases male virility and sexual prowess during sexual activities. The plant parts have been traditionally used for its antimalarial, aphrodisiac, anti-diabetic, antimicrobial and antipyretic (fever reduction) activities, which have also been proved scientifically. Moreover, the compounds of the roots of this plant are reported to have aphrodisiac and testosterone enhancing effects in the rats. Therefore, Eurycoma longifolia can be found in many testosterone boosters.
Eurycoma Longifolia Jack (Longjack) and Low Testosterone
There are plenty of animal studies using the rat as model showing that Eurycoma longifolia Jack (Longjack) supplementation increased sexual motivation and performance including increased serum testosterone concentrations. It seems that Eurycoma longifolia produces a dose-dependent, recurrent and significant increase in the episodes of penile reflexes and increase in mounting frequency. There are, however, some studies that have failed to measure significant improvements in testosterone levels in normal rats after Eurycoma longifolia Jack supplementation.
One very recent study in rats by Solomon and associates further investigated aphrodisiac effects of Eurycoma longifolia Jack (Longjack). Because previous studies were limited to only few sperm parameters or testosterone concentration, Solomon and colleagues investigated the in vivoeffects of Eurycoma longifolia Jack (Longjack) on body and organ weight as well as functional sperm parameters in terms of safety and efficacy in the management of male infertility. After 14 days of force-feeding researchers weighted sexual organs and muscles and assessed testosterone concentration, sperm concentration and mitochondrial membrane potential. There was no changes in organ weights, testosterone concentration increased by 30.2% (p = 0.0544), muscle weight increased but not significantly, sperm concentration, motility and vitality increased significantly, mitochondrial membrane potential improved by 25.1%(P = 0.0765). Bin-Seng Low and colleagues also reported increase in plasma testosterone levels when male rats were fed with 25 mg/kg of Eurycoma longifoliaJack extract. Compared to placebo significant difference in testosterone levels were evident at day 26 (p<0.05) and day 52 (P<0.01) but not at day 104. In contrast, the plasma estrogen level was significantly lower than that of the untreated control.
The only human studies that are accessible online are in subjects with hypogonadism (diminished functional activity in the testes). 76 males were given 200 mg of a standardised water-soluble extract of Eurycoma longifolia Jack (Longjack) for 1 month. Results show that treatment of hypogonadic patients with Eurycoma longifolia Jack extract significantly (P < 0.0001) improved the aging male syndrome score as well as the serum testosterone concentration. Interestingly one very recent study  in healthy males who received 200mg of Eurycoma longifolia and 100 mg of Polygonum minus demonstrated an increase in total testosterone (10.36%) but decrease in free testosterone (active testosterone). Which suggests that these two compounds (alone or in combination) may also increase sex-hormone binding globulin or increased metabolism and breakdown of free testosterone. When patients (75 completed one full cycle) suffering from infertility were given 200 mg of the Eurycoma longifolia Jack (Longjack) extract daily for 9 months sperm quality was significantly improved, allowing for 11 (14.7%) spontaneous pregnancies.
Eurycoma longifolia Jack (Longjack) seems to increases testosterone levels in rats with hypogonadism (or clinical signs of hypogonadism). There is lack of studies on otherwise healthy rats. The only evidence on humans is also in persons with hypogonadism, other studies that claim increases of testosterone in healthy humans cannot be accessed online, are short and are of questionable quality.
According to WebMD.com : “Some of the chemicals seem to affect how the body produces the hormone testosterone. Research in animals and humans suggests Eurycoma longifolia might increase testosterone in the body.” However, there is a possibility that Eurycoma longifolia may only work when there is testosterone deficiency as Eurycoma longifolia supplementation to normal male rats failed to elevate the testosterone levels compared to nontreatment rats.
When female rats were injected with testosterone, which lead to irregular estrous cycle thereby affecting the ovarian function, 50mg/kg of a standardized quassinoid-rich extract (TAF 273) of Eurycoma root was able to improve irregular estrous cycles with less follicular morphological damage. It appears that potent anti-estrogenic effect of Eurycoma (especially quassinoid Eurycomanone) was able to ameliorate the adverse effects of testosterone injection.
Effects on Stress
It seems that daily supplementation with tongkat ali root extract improves stress hormone profile (reduced cortisol levels by 16% and increased testosterone status by 37%) . Treatment group also exhibited significant improvements for tension (−11%), anger (−12%), and confusion (−15%) compared to placebo.
Eurycoma longifolia Jack is normally recommended to be administered by men at the dose of 200–400 mg daily. The calculated acceptable daily intake is up to 1.2 g/60 kg adult/day. However, toxicological reports are incomplete and insufficient for demonstrating either long-term safety or prediction of acceptable daily intake.
Eurycoma longifolia Jack (Longjack) Side Effects
Even though toxicity and safety evaluation studies have been pursued, there is still not enough information to know for sure if Eurycoma longifolia is safe. However, recent study in animals reported that Eurycoma longifolia appears safe for possible treatment of male infertility and ageing male problems. Results of one recent in vitro study indicate that the Eurycoma longifolia extract has no deleterious effects on sperm functions at therapeutically used concentrations. However, at very high doses, in may have harmful effects in vitro. 600 mg/kg of Eurycoma longifolia Jack did not cause any adverse effects in a human clinical study. A recent study reported that Eurycoma longifolia might be a safe alternative to testosterone replacement therapy. Another very recent study in healthy males reported no liver and kidney abnormalities after supplementation with 300mg combination of Eurycoma longifolia and Polygonum minus for 12 weeks. In rats, LD50 for Eurycoma has been found to be greater than 5000mg/kg.