So since Viking had me up all night the last few nights. here I go..

Mesterolone is an orally active, 1-methylated DHT. Like Masteron, but then actually delivered in an oral fashion. DHT is the conversion product of testosterone at the 5-alpha-reductase enzyme, the result being a hormone that is 3 to 4 times as androgenic and is structurally incapable of forming estrogen. One would imagine then that mesterolone would be a perfect drug to enhance strength and add small but completely lean gains to the frame. Unfortunately there is a control mechanism for DHT in the human body. When levels get too high, the 3alpha hydroxysteroid dehydrogenase enzyme converts it to a mostly inactive compound known as 3-alpha (5-alpha-androstan-3alpha,17beta-diol), a prohormone if you will. It can equally convert back to DHT by way of the same enzyme when low levels of DHT are detected. But it means that unless one uses ridiculously high amounts, most of what is administered is quite useless at the height of the androgen receptor in muscle tissue and thus mesterolone is not particularly suited, if at all, to promote muscle hypertrophy.

Proviron has four distinct uses in the world of bodybuilding. The first being the result of its structure. It is 5-alpha reduced and not capable of forming estrogen, yet it nonetheless has a much higher affinity for the aromatase enzyme (which converts testosterone to estrogen) than testosterone does. That means in administering it with testosterone or another aromatizable compound, it prevents estrogen build-up because it binds to the aromatase enzyme very strongly, thereby preventing these steroids from interacting with it and forming estrogen. So Mesterolone use has the extreme benefit of reducing estrogenic side-effects and water retention noted with other steroids, and as such still help to provide mostly lean gains. Its also been suggested that it may actually downgrade the actual estrogen receptor making it doubly effective at reducing circulating estrogen levels.

The second use is in enhancing the potency of testosterone. Testosterone in the body at normal physiological levels is mostly inactive. As much as 97 or 98 percent of testosterone in that amount is bound to sex hormone binding globulin (SHBG) and albumin, two proteins. In such a form testosterone is mostly inactive. But as with the aromatase enzyme, DHT has a higher affinity for these proteins than testosterone does, so when administered simultaneously the mesterolone will attach to the SHBG and albumin, leaving larger amounts of free testosterone to mediate anabolic activities such as protein synthesis. Another way in which it helps to increase gains. Its also another part of the equation that makes it ineffective on its own, as binding to these proteins too, would render it a non-issue at the androgen receptor.

Thirdly, mesterolone is added in pre-contest phases to increase a distinct hardness and muscle density. Probably due to its reduction in circulating estrogen, perhaps due to the downregulating of the estrogen receptor in muscle tissue, it decreases the total water build-up of the body giving its user a much leaner look, and a visual effect of possessing "harder" muscles with more cuts and striations. Proviron is often used as a last-minute secret by a lot of bodybuilders and both actors and models have used it time and again to deliver top shape day in day out, when needed. Like the other methylated DHT compound, drostanolone, mesterolone is particularly potent in achieving this feat.

Lastly Proviron is used during a cycle of certain hormones such as nandrolone, with a distinct lack of androgenic nature, or perhaps 5-alpha reduced hormones that don't have the same affinities as DHT does. Such compounds, thinking of trenbolone, nandrolone and such in particular, have been known to decrease libido. Limiting the athlete to perform sexually being the logical result. DHT plays a key role in this process and is therefore administered in conjunction with such steroids to ease or relieve this annoying side-effect. Proviron is also commonly prescribed by doctors to people with low levels of testosterone, or patients with chronic impotence. Its not perceived as a powerful anabolic, but it gets the job done equally well if not better than other anabolic steroids making it a favorite in medical practices due to its lower chance of abuse.

Mesterolone is generally well liked nonetheless as it delivers very few side-effects in men. In high doses it can cause some virilization symptoms in women. But because of the high level of deactivation and pre-destination in the system (albumin, SHBG, 3bHSD, aromatase) quite a lot of it, if not all simply never reaches the androgen receptor where it would cause anabolic effects, but also side-effects. So its relatively safe. Doses between 25 and 250 mg per day are used with no adverse effects. 50 mg per day is usually sufficient to be effective in each of the four cases we mentioned up above, so going higher really isn't necessary. Unlike what some suggest or believe,

Its not advised that Proviron be used when not used in conjunction with another steroid, as it too is quite suppressive of natural testosterone, leading to all sorts of future complications upon discontinuation. Ranging from loss of libido or erectile dysfunction all the way up to infertility. One would not be aware of such dangers because Proviron fulfills most of the functions of normal levels of testosterone.

Masteron (Drostanolone Propionate) is perhaps one of the more 'exotic' androgenic / anabolic steroids (anabolic steroids) that may be used by an athlete. Originally it was developed and used as an anti-estrogen (under the name Masteril) for the treatment of breast cancer. It was largely used in combination with the selective estrogen receptor modulator (Selective Estrogen Receptor Modulator) Tamoxifen (aka Nolvadex) for the treatment of breast cancer, and did give a significant decrease in estrogen levels in women undergoing such treatment. It is not much used these days for such purposes, for varying reasons, however for many athletes including competitive bodybuilders in particular; Masteron remains a rather unsung favourite of AS medicines.

The fact that Masteron was being used as an anti-estrogen goes to suggest quite a lot about some properties Masteron possesses. Masteron is a derivative of dihydrotestosterone (dihydrotestosterone) and does not convert to estrogen through means of aromatisation. It is thought that the anti-estrogenic properties of Masteron may be in part to do with either an inhibition in some way of the aromatase enzyme or an interaction with estrogen itself in a way which blocks receptor binding of the estrogen. Either way, this would put Masteron as a useful tool for the AS user who uses compounds that convert to estrogen (which most AS users do, considering testosterone is the main basis of most cycles). By inhibiting the aromatase enzyme, Masteron would be in effect blocking the conversion of free testosterone to estrogen by the aromatisation pathway. This would not only serve to marginally increase the amounts of active free testosterone in circulation (thus giving a greater effect of the testosterone over a Masteron-free system), but it would also negate the side-effects that result from high levels of estrogen due to aromatisation. Such side effects include the development of gynecomastia and water retention/bloating. Conversely, if Masteron actually blocks the binding of estrogen to the estrogen receptor (ER) in some way, although aromatisation of testosterone may occur, its effects would be limited due to the inability of the estrogen to bind to the ER. Thus through this mechanism, the effects of excess estrogen production through aromatisation would also be limited by use of Masteron.

Masteron is derived from Dihydrotestosterone (DHT) but with an added 2a-methyl group. It is therefore structurally incapable of conversion to either estrogen or progesterone. Judging from the literature I’ve seen, the 2a-methyl group would also seem to make it less of a substrate for 3a-HSD (3alpha-Hydroxysteroid dehydrogenase), and maybe the beta version of that enzyme as well. This is good because those enzymes serve to, in some ways, inactivate DHT. They are responsible for the inactivation of DHT into a less-anabolic hormone. They are also the reason why injectable DHT is not produced as an anabolic on its own.

Masteron has been used to treat certain forms of breast cancer (1)(2), and for this, it’s reasonably effective. It was, however, been largely replaced by first through third generation Aromatase Inhibitors such as from Nolvadex to Letrozole. However, in at least one study, Masteron & Tamoxifen produced better results than Chemotherapy with regards to producing immediate objective responses from patients (3). Another rarely spoken about property of Masteron is that it is actually a weak (though useful) anti-estrogen, and that’s where a lot of its “hardening” properties probably come from, and why it’s so useful in precontest cycles. Masteron may actually interact with the aromatase enzyme to inhibit aromatization (conversion/metabolism) of other steroids into estrogen.

Reference
Anabolic Steroids - The Ultimate Research Guide
FDA study of Mast From 1968 to 1972, for breast cancer
Department of Obstetrics & Gynaecology, St. George's Hospital Medical School London, U.K.
PMID: 2892728 [PubMed - indexed for MEDLINE]
Effect of non aromatizable androgens on LHRH and TRH responses in primary testicular failure.
Spitz IM, Margalioth EJ, Yeger Y, Livshin Y, Zylber-Haran E, Shilo S.