Androstenedione

Androstenedione is a hormone precursor secreted by the adrenal glands, and found in meat and (oddly) the pollen of Scotch pine. It's been used for quite some time by athletes needing endurance or porn stars looking for a different sort of endurance. The latter also benefit from increased sperm production, by the way.
The prohormone affects the body by being transformed to testosterone by the liver enzyme 17beta hydroxysteriod dehydrogenase. (What?) Since androstenedione is only one metabolic step away from testosterone, the body converts it rather quickly and easily to that libido-driving hormone, unlike most other hormone precursors, which are frankly a little overhyped by well-meaning pseudoscience. Many are mentioned on this site. Androstenedione's effects are dramatic enough to warrant notice even by the skeptical, though. According to the German patent for androstenedione, 50mg given orally to men raised plasma testosterone levels at least 140%, and a 100mg dose more than tripled the subject's initial levels. Most tablets available supply 100mg of androstenedione, but a more moderate dosage of 25mg is available and is considered more cautious.
If taken in the recommended dose, it should not shut down natural hormone production. Since the prohormone conversion only lasts about 3 hours, there is insufficient time to signal the adrenal gland to shut down. Obviously, taking 100mg every three hours could result in problems, but in general there are no side effects. Even a blood test later in the same day would probably not register anything unusual, as androstenedione is always present in the body anyway. Blood testosterone levels start rising about 15 minutes after taking androstenedione, and stay elevated for around three hours. A peak in the levels is seen around 60 to 90 minutes after ingestion.
Those wishing to delve further into this controvertial substance's literature will soon note that there are several varieties of androstenedione. My suggestions for the specific chemicals you may find useful are 5-Androstenediol, 19-NorAndrostenedione and 19-Nor5- Androstenediol.