Tibolone, also known as 7α-methylnoretynodrel, as well as 7α-methyl-17α-ethynyl-19-nor-δ 5(10) -testosterone or as 7α-methyl-17α-ethynylestr-5(10)-en-17β-ol-3-one, is a synthetic estrane steroid and a derivative of testosterone and 19-nortestosterone .   It is more specifically a derivative of norethisterone (17α-ethynyl-19-nortestosterone) and is a member of the estrane subgroup of the 19-nortestosterone family of progestins.     Tibolone is the 7α-methyl derivative of the progestin noretynodrel (17α-ethynyl-δ 5(10) -19-nortestosterone).  Other steroids related to tibolone include the progestin norgesterone (17α-vinyl-δ 5(10) -19-nortestosterone) and the anabolic steroids trestolone (7α-methyl-19-nortestosterone) and mibolerone (7α,17α-dimethyl-19-nortestosterone). 
Testosterone and its derivate are made synthetically pharmaceutically as drugs for many different treatments. Pharmaceutical research of anabolic steroid is not focused a lot on muscle grow in healthy individuals but only in some aging related diseases like sarcopenia – the loss of skeletal muscle mass and strength as a result of aging. Osteoporosis is one of the aging diseases that is also sometimes treated with anabolic steroids. In all these treatments, serious hormonal misbalance is highly possible, and a reaction of other, not targeted tissues. Some of the side effects using anabolic steroids can lead to liver conditions like peliosis hepatis – blood-filled cysts replacing healthy liver cells. This condition is often diagnosed late when it is already life threatening to treat liver failure present or intra-abdominal bleeding start.
The side effects of Letrozole can include those surrounding cholesterol, and this is an issue some steroid users tend to have a hard time with. The overwhelming data shows AI’s like Letrozole have very little affect on cholesterol, and if they do have any affect it’s normally insignificant. However, such data and studies rarely take into account the inclusion of anabolic steroids, specifically aromatizing anabolic steroids, and this changes the game. When examining the side effects of Letrozole, it’s important to understand the inclusion of anabolic steroids can change things. Many anabolic steroids have the ability to negatively affect cholesterol; the steroid in question will determine the total potential affect. Oral steroids are normally the harshest as they often significantly increase LDL cholesterol (bad cholesterol) and suppress HDL cholesterol (good cholesterol). Then we have injectable steroids , the their affects on cholesterol can also vary greatly. Trenbolone can have a strong, negative impact on both HDL and LDL. Testosterone can also have an impact on HDL, but normally not very strong. Hormones like Nandrolone will typically fall somewhere in the middle. With the above information in play, although it cannot significantly affect cholesterol on its own, what happens when Letrozole is used with anabolic steroids, specifically aromatizing steroids? Data shows when testosterone is used in therapeutic doses the relative affect on cholesterol is statistically insignificant. When doses are increased to supraphysiological levels, HDL suppression may occur but should not be at a true level of concern. The same data shows things, however, change greatly when the same testosterone is used with an AI like Letrozole. Data shows that a 20-25% reduction in HDL cholesterol is very possible with therapeutic or supraphysiological doses of testosterone. For this reason, a cholesterol friendly diet is important, one that’s rich in omega fatty acids, low in simple sugars and saturated fats and a lifestyle that promotes regular cardiovascular activity.