Longjack, also known as Tongkat Ali, is a Southeast Asian plant root used primarily to support testosterone levels, male fertility, and stress management. It has a long history in traditional Malaysian medicine and has become one of the more popular testosterone-related supplements worldwide. The active compounds in the root, particularly a group called quassinoids, are responsible for most of its effects.
How Longjack Works in the Body
The root of the longjack plant contains dozens of bioactive compounds, but quassinoids make up the largest share. The most important of these is eurycomanone, which researchers consider the primary driver of longjack’s health effects. Other notable quassinoids include eurycomanol and dihydroeurycomanone. These compounds appear to influence the balance between cortisol (a stress hormone) and testosterone, which is why longjack shows up in supplements marketed for hormonal support, energy, libido, and athletic performance.
Effects on Testosterone
The most common reason people take longjack is to raise testosterone. A systematic review and meta-analysis of clinical trials found that supplementation significantly improved total testosterone in both healthy men and men with clinically low levels. The size of the increase varied depending on the population studied.
Men with low testosterone saw the most dramatic shifts. In one trial of 76 men with late-onset hypogonadism, average total testosterone rose from 163 ng/dL to 240 ng/dL. Another study of men aged 40 to 59 with androgen deficiency showed levels climb from about 278 ng/dL to 400 ng/dL. In younger, healthy men aged 18 to 30, the increase was more modest, from around 802 ng/dL to 924 ng/dL. A small study of trained young men found testosterone roughly doubled over five weeks, though the sample size was only nine people.
Results for free testosterone (the portion your body can actually use) are less consistent. About half of the studies measuring free testosterone found a significant increase, while the other half did not. So while total testosterone tends to rise across most trials, the effect on free testosterone is less reliable.
Male Fertility
Longjack has shown promising results for men dealing with unexplained infertility. A study of 75 men with idiopathic male infertility found significant improvements across all major semen parameters after several months of supplementation. Sperm concentration increased by about 65% from baseline, and the percentage of sperm with normal shape nearly doubled, improving by roughly 95%. Sperm motility (how well sperm swim) also improved early in treatment, rising from about 45% to 50% in the first cycle.
Among the 11 men in that study whose partners became pregnant, the improvements were even more pronounced. Their sperm motility jumped from 44% at baseline to nearly 59% at follow-up, and normal sperm morphology rose from about 10% to over 16%. These are meaningful changes for couples trying to conceive, particularly when no clear cause of infertility has been identified.
Stress and Mood
One well-cited trial looked at moderately stressed adults taking longjack for four weeks. The supplement group experienced a 16% reduction in salivary cortisol and a 37% increase in testosterone compared to placebo, improving the overall cortisol-to-testosterone ratio by 36%. That hormonal shift translated into measurable mood changes: tension dropped 11%, anger decreased 12%, and confusion improved by 15%. Depression, energy levels, and fatigue scores, however, didn’t change significantly compared to placebo.
This cortisol-lowering effect is one of the more distinctive aspects of longjack. Many testosterone supplements focus solely on boosting testosterone production, but longjack appears to work partly by reducing the hormonal impact of stress, which indirectly supports healthier testosterone levels.
Athletic Performance and Body Composition
Despite its reputation in the fitness world, the evidence for longjack as a muscle-building or fat-burning supplement is surprisingly weak. A controlled trial of exercise-trained men and women found no significant differences between longjack and placebo for lean body mass, fat mass, body fat percentage, or handgrip strength. The supplement group lost about half a kilogram of fat on average, but so did the placebo group at a similar rate.
There is one interesting nuance. When longjack was combined with resistance training in a separate study, the combination improved peak power output more than resistance training alone. But longjack without exercise produced no changes in muscular strength or power. In other words, it may offer a small edge when paired with a structured training program, but it won’t do the work for you.
A 12-week trial did find that men with a BMI over 25 experienced a significant reduction in abdominal fat while taking 300 mg per day, along with an 11 to 14% increase in sexual desire. So for overweight men specifically, there may be modest benefits for body composition, though these results need more replication.
Erectile Function
Longjack is frequently sold as a natural remedy for erectile difficulties, but the clinical evidence here is limited. A meta-analysis of randomized controlled trials found only two qualifying studies (139 total participants), and the overall results showed no significant improvement in erectile function scores compared to placebo. However, a subgroup analysis revealed something worth noting: men who started with lower baseline erectile function did see a meaningful improvement, while those with milder issues did not. This suggests longjack may help men with more noticeable erectile difficulties but is unlikely to enhance function in men who are already performing normally.
Safety and Side Effects
Longjack has a relatively clean safety profile based on the available toxicology data. Animal studies found the lethal dose to be extremely high, more than 6 grams per kilogram of body weight, which is far above any supplement dose. A 13-week study using doses up to 2 grams per kilogram per day found no adverse effects on body weight, blood markers, organ function, or tissue health. The calculated acceptable daily intake for adults is up to 1.2 grams per day.
The bigger safety concern is product quality. A screening of commercial Tongkat Ali products found that 26% contained mercury levels between 0.53 and 2.35 parts per million, exceeding the Malaysian safety limit of 0.5 ppm for traditional medicines. Because longjack is sold as a dietary supplement rather than a drug in most countries, quality control varies widely between brands. Choosing products that provide third-party testing results or standardized eurycomanone content (typically 1% to 3%) reduces the risk of contamination.
Typical Dosing
Most clinical trials have used standardized root extracts at doses between 200 and 400 mg per day, taken for periods ranging from 4 to 12 weeks. Some studies used weight-based dosing around 1.7 mg per kilogram of body weight. The extract used in the majority of positive trials is a hot-water extraction standardized to its eurycomanone content. Raw root powder requires much higher doses, but standardized extracts are more commonly available and better studied.