THG, also known as tetrahydrogestrinone, is a synthetic steroid that comes from the female infertility drug Gestrinone. Since the Food & Drug Administration do not approve Gestrinone for sale, THG has also been banned.
Gestrinone itself is not very anabolic (or muscle producing). But adding four hydrogen atoms -- the Tetra in THG -- turns it into a potent steroid. In a review of the drug, William Llewellyn, author of the book Anabolics 2004, writes:
"The results from a cycle of tetrahydrogestrinone are likely to be quite high quality. The steroid imparts pure lean muscle tissue and strength gains instead of the sheer mass we would expect from bulking agents like testosterone, Dianabol or Anadrol. It is much more like Winstrol or Anavar, which makes THG a steroid ideally suited for athletes competing in speed sports where it is not desirable to carry around unnecessary water weight."