Tetrahydrogestrinone (THG), often referred to as The Clear, is an anabolic steroid developed by Patrick Arnold. It has affinity to the androgen receptor and the progesterone receptor, but not to the estrogen receptor.
Developed completely in secret by Patrick Arnold, the drug was used in performance enhancement without having been studied for side-effects. The drug has been considered a designer drug, closely related to the banned anabolic steroids gestrinone and trenbolone, and was referred to as an unapproved new drug by the Food and Drug Administration (FDA) in 2003 before the subsequent scheduling of it in 2005. A synthesis was devised to ensure a source of material for comparison. The drug has never entered legitimate medical use, although some studies have been made of its properties and toxicity. Concerns have also been raised about its potential use in horse-racing and other veterinary contexts.

Chemical Name	(8S,13S,14S,17S)-13,17-diethyl-17-hydroxy-1,2,6,7,8,14,15,16-octahydrocyclopenta[a]phenanthren-3-one
Synonyms	Tetrahydrogestrinone;17H;BIDD:ER0580;17-HYDROXY-18A-HOMO-19-NOR-17ALPHA-PREGNA-4,9,11-TRIEN-3-ONE;Tetrahydrogestrinone (THG);Clear;Collapse
CAS No.	618903-56-3
Molecular Formula	C21H28O2
Molecular Weight	312.44600
PSA	37.30000
LogP	4.49960

Density	1.13g/cm3
Boiling Point	505.7ºC at 760 mmHg
Flash Point	214.2ºC
Refractive Index	1.583