Tongkat Ali is a tree native to Indonesia, Malaysia and Thailand. The part of the tree used medicinally is mainly the root. 

Tongkat Ali was used for many years by men to increase sexual desire, libido, sexual performance and to treat erectile dysfunction. Tongkat Ali works by increasing the levels of male hormone testosterone. Testosterone is primarily responsible for the growth and development of male reproductive organs including the penis, testicles, scrotum, prostrate and seminal vesicles. Normal testosterone levels maintain energy level, mood, fertility and sexual desire. Because of its testosterone-enhancing properties, Tongkat Ali is used by body builders to increase muscle mass and strength. 

Biological Effects :- 

A 2010 ethnopharmacological inventory study on Eurycoma longifolia stated: "The plant parts have been traditionally used for its antimalarial, aphrodisiac, anti-diabetic, antimicrobial and antipyretic activities…

Safety :-

Overdose will exhibit side effects such as insomnia, anxiety and restlessness. Vital Herbs Tongkat Ali can be taken up to 4 capsules per day, however 1 capsule per day is sufficient enough to see significant results in 3 - 4 days. As Tongkat Ali increases testosterone levels it should not be used by men with breast cancer or prostate cancer, diabetes mellitus, heart diseases, kidney disease, liver disease or sleep apnea. 

Drug Interactions :-

Tongkat Ali should not be used by people taking immuno-suppressant drugs.

Unlike drugs available in the market, Tongkat Ali is 100% natural. Tongkat Ali does not give a 3-4 hour "fix" but builds up the bodies libido for extended periods. 

You may see the following links for more information about Tongkat Ali.

http://en.wikipedia.org/wiki/Eurycoma_longifolia

http://altmedicine.about.com/od/herbsupplementguide/a/tongkat_ali.htm