Numerous scientific studies have shown that properly concentrated tongkat ali extract (not the fakes and not the junk commonly sold as supplements) will raisetestosterone levels. The rise of testosterone during puberty is what causes penis growth in boys . Depressing testosterone synthesis throughexogenous hormones in bodybuilders causes penile and testicular shrinkage  . All of this is well documented in the scientific literature, and even in college textbooks.
Tongkat ali has been shown to raise testosterone. It is alleged that it does so by stimulating the Leydig cells of the testiclesto produce more testosterone. This is different from the action of testosterone products, which act like the body’s own testosterone and therefore causes theLeydig cells to shut down their production of the hormone. For this reason, all exogenous testosterone products, including anabolic steroids, cause testicular and penile shrinkage.
I do think that this is not sufficiently emphasized by anti-doping campaigns: athletes who dope with testosterone and anabolic steroids will cause their penises to become as small as their thumbs or little fingers, and unfortunately, if exogenous testosterone or anabolic steroids are taken for some time, the loss in size can be permanent. The effect of tongkat ali is the opposite. Because tongkat ali stimulates the body’s own testosterone production in the Leydig cells of the testicles, the testicles and surrounding tissue will grow in size. (During puberty, it’s a spur in Leydig cell activity, which causes a boy’s sexual organs to become the size of a man’s. Furthermore, testosterone over-production endocrine disorders during adolescence cause oversized male genitalia .) While a large number of men may consider increased penile and testicular size a benefit in its own right, the mass gain also influences sexual function, especially erections and ejaculations. The erectile and ejaculatory support of tongkat ali is, so it appears, a direct consequence of then increased size of male accessory sexual structures.
1 Science Encyclopedia, Puberty – Male Puberty
2 Dr. Abraham Kryger, MD, DMD, Penis Size and Enhancement
3 Encyclopedia of Children’s Health, Puberty
4 Mike Cernovich, The Truth About Anabolic Steroids and Testosterone
5 Herballove Editorial, Penis Shrinkage – Caused by High Levels of Stress Hormones
6 Robert M. Sargis MD, PhD, An Overview of the Testes
7 Michael Westwood, MB ChB MRCP(UK) FRCPC1 and Jorge Pinzon, MD FRCPC2, Adolescent male health