Thu HE, Mohamed IN, Hussain Z, Jayusman PA, Shuid AN. Eurycoma longifolia as a potential adoptogen [sic] of male sexual health: a systematic review on clinical studies. Chin J Nat Med. January 2017;15(1):71-80.

Tongkat ali (Eurycoma longifolia, Simaroubaceae) has been the subject of numerous in vivo and human clinical studies regarding its potential in improving male sexual health. The authors performed a literature search and, of 150 articles located, eventually chose seven human studies published between 2000 and 2014 for review. Most studies included were randomized placebo-controlled trials (RCTs).

Sexual health is a major factor in the quality of life for men and women. Sexual activity requires coordination among several biological systems. For men, creating and maintaining an adequate erection for sexual activity requires specialized responses of penile nerves and blood vessels. Sexual dysfunction, experienced by perhaps one in 10 men, is a leading cause of medical and psychological distress and should be evaluated by a health care specialist. The most common male sexual concerns include erectile dysfunction (ED); premature ejaculation (PE); male infertility (MI); and low libido/low levels of testosterone (LLT), the primary male sex hormone. Prevalence of ED is greatly affected by age. Of an estimated 30 million men in the United States with ED, 18% are from 50-59 years of age, with a prevalence of 50-75% in men in their 70s and 80s worldwide. Potential causes of ED include neurological, vascular, hormonal, pharmacological, and psychological issues, with therapeutic success depending largely on accurate diagnosis of these causes. Lifestyle changes such as cessation of tobacco (Nicotiana tabacum, Solanaceae) use, reduced alcohol intake, healthy diet, and exercise are often the first measures recommended. Using a genitalia-friendly bicycle seat also is suggested where this may be useful. Pharmacological remedies for ED are mostly phosphodiesterase-5 (PDE-5) inhibitors that regulate blood flow to the penis but may have adverse effects. Causes of PE, affecting 20-30% of adult men, may be related to central control of ejaculation, innervation of the seminal tract, or sensory innervations of the genitalia or prostate gland. It could be an early indication of diabetes mellitus (diabetes type 2), may develop after surgery for benign prostatic hyperplasia, or may be due to some medications. MI, a multifactorial disorder, is responsible for about 20% of infertile couples. Numerous health issues, medical problems, and lifestyle choices are associated with MI. LLT is a common issue by about age 60-65.

Tongkat ali is widely used in Malaysia, Thailand, and Indonesia to treat male sexual disorders and MI. Its aphrodisiac effects improve libido, restore erectile function, and stimulate testosterone production. Its possible mechanisms of action include increasing conversion of pregnenolone to progesterone, 5-dehydroepiandrosterone (DHEA), and testosterone in the corpora cavernosa. Extracted data from the studies reviewed indicate that eurypeptide, found in tongkat ali root, exerts aphrodisiac effects by boosting testosterone. Some studies also indicate tongkat ali's potential as an energy enhancer, potentially affecting sexual health.

Besides RCTs, studies reviewed included multiple-cohort trials and pilot studies. Sample sizes varied from 13-350, and subjects' ages ranged from 30-72 years. Penile erection, penile hardness, male sexual libido, MI, and determination of testosterone level were the primary concerns in most studies included. Two RCTS that evaluated effects of tongkat ali on ED reported "remarkable" or "noteworthy" improvement in erectile function, sexual intercourse attempts, and sexual well-being of men in the active groups. The herb's effects on MI were evaluated in two Malaysian studies, including an RCT and a placebo-controlled trial. Significant effects were reported including in semen volume, sperm concentration, percentage of sperm having normal morphology and motility, libido, sexual activity and satisfaction, and, in a placebo-controlled trial lasting nine months, 11 (about 15%) spontaneous pregnancies. Six studies investigated tongkat ali's effects on LLT. With different populations and study parameters, all reported significant improvements in testosterone levels, sexual performance, and sexual satisfaction. One found tongkat ali a "remarkable" male adaptogen in restoring serum testosterone compared to testosterone replacement therapy, improving male libido in patients in their 60s who had, at baseline, 40-50% less testosterone than younger ones.

Despite its wide recognition in Southeast Asia for treatment of male sexual dysfunction, most research on tongkat ali has been based on in vivo animal studies. This review did not include studies published in languages other than English, and these may have contained relevant information. Pharmacodynamic and pharmacokinetic studies are needed. The exact mechanisms of tongkat ali's aphrodisiac, testosterone-boosting, and libido-boosting effects are not well known. Some researchers propose that its aphrodisiac effects are due to its glycoprotein and eurypeptide content (via increasing testosterone levels). Others propose that tongkat ali exerts direct benefits on corpora cavernosa and seminal vesicles' muscle tone. Increased sperm concentrations may be attributable to suppression of apoptosis of sperm in the presence of antisperm antibodies. Some researchers correlate tongkat ali's benefits to male fertility to its antioxidant properties. Testosterone levels may be improved by release from the bound form of the male sex hormone and/or reducing its metabolism. Where safety has been studied, there have been no significant changes in liver or kidney function in subjects supplemented with tongkat ali. Although showing great promise as a male sexual adaptogen, current evidence to support tongkat ali's clinical use for other human health concerns needs to be further explored.