Mesterolone is the brand name for oral androgen mesterolone (1-methyl dihydrotestosterone) by Bayer (formerly Schering). Similar to dihydrotestosterone, mesterolone is a strong androgen with weak anabolic activity. Due to its similarity to dihydrotestosterone, mesterolone is rapidly reduced to inactive diol metabolites in muscle tissue where the concentration of 3-hydroxysteroid dehydrogenase is high. In fact, due to its high affinity for plasma-binding proteins such as SHBG, mesterolone can enhance its steroidal activity by displacing bound steroids into a free, unbound state. Among athletes, mesterolone is primarily used to increase androgen levels during dieting or competition preparation and is used as an anti-estrogen due to its ability to antagonize aromatase.

Product History:

According to company records, Schering developed mesterolone in 1934 as one of the earliest synthetic anabolic steroids for clinical use in "hormone-related diseases." Thus, mesterolone was developed around the same time as methyltestosterone (1935) and testosterone propionate (1937), making it one of the oldest agents. Mesterolone has a long record of efficacy and safety and is still widely used in clinical medicine today. It is typically used in men to treat physiological and psychological decline caused by aging and low levels of androgen, low libido due to androgen deficiency, hypogonadism (in males before and after puberty), and infertility (in specific cases, mesterolone can improve the quality and quantity of sperm).

Mesterolone as a fertility aid can be controversial, as synthetic anabolic/androgenic steroids are generally associated with infertility. This is one of the misunderstandings athletes have about testosterone because its androgenicity, at therapeutic doses, can minimize suppression of gonadotropins rather than increasing luteinizing hormone output. Since it does not suppress gonadotropins significantly, the drug can supplement the androgens necessary for sperm production. Androgens are known to directly stimulate sperm production and also affect the transport and maturation of sperm by reacting with the epididymis, tubular structures, and seminal vesicles. Thus, these hormones are not purely suppressive. Mesterolone appears to have a unique positive impact on some men's fertility due to its potential stimulation of sperm quantity and quality not being offset by gonadotropin suppression.

Mesterolone is widely produced and sold by Bayer (formerly Schering), and is available in over thirty countries. Although Bayer markets it under different trade names in other markets, such as mestoranum and provironum, the most common brand name is Proviron. Additionally, in recent years, other manufacturers have also sold mesterolone under brand names like Pluriviron (Asche, Germany), Vistimon (Jenapharm, Germany), and Restore (Brown & Burke, India). Despite its safe and effective history of use, the FDA has never approved its entry into the US market. Today, mesterolone can still be seen in Western countries. Bayer remains its main global supplier, but in very rare cases, other brand names might be seen.

Available Forms:

Mesterolone is extensively used in the human medicinal market. Composition and dosage may vary by country and manufacturer; the formulation is typically available in tablets containing 25 mg or 50 mg of the steroid.

Structural Characteristics:

Mesterolone is a modified form of dihydrotestosterone. Its modification at carbon 1 by adding a methyl group helps protect the hormone from destruction by the liver during oral administration. The same structural modification is used in oral mesterolone tablets. The specific alkylation at this position slows down the metabolism of the steroid during its first pass through the liver, though not as significantly as C-17α alkylated steroids. Mesterolone's overall bioavailability remains much lower than that of C-17α alkylated oral steroids, but it has enough resistance to achieve therapeutically beneficial blood levels. Mesterolone also has a very strong binding affinity for Sex Hormone Binding Globulin, which can be used to replace other steroids with weaker binding to SHBG.

Side Effects (Estrogenic):

Mesterolone cannot be aromatized by the body and has no estrogenic activity. Therefore, anti-estrogens are not necessary when using mesterolone. It does not cause gynecomastia, water retention, or other estrogen-related side effects.

It is believed that mesterolone can act as an anti-aromatase in the body, preventing or slowing the conversion of steroids into estrogen. The effect is somewhat similar to that of Arimidex, though not as profound. Mesterolone's anti-estrogenic properties are not unique, as many other steroids have shown similar activity. For instance, dihydrotestosterone and clostebol have been successfully used to treat gynecomastia and breast cancer due to their strong androgenic and potential anti-estrogenic properties. Nandrolone is considered to reduce aromatase activity because it is more resistant to estrogen conversion (nandrolone aromatizes most actively in the liver). The anti-estrogenic properties of all these compounds might be due to their competition with other substrates for binding to aromatase. Since the enzyme can bind to the steroid but cannot alter it, it is temporarily blocked from interacting with other hormones.

Side Effects (Androgenic):

As an androgenic steroid, mesterolone carries androgenic side effects, especially when used at high doses. Common side effects may include oily skin, acne, and body/facial hair growth. Anabolic/androgenic steroids may also exacerbate male pattern baldness. Female users should be aware of potential side effects including deepening of the voice, menstrual irregularities, changes in skin texture, facial hair growth, and clitoral enlargement. Since mesterolone is not metabolized by 5-α reductase, it is not significantly affected when used with finasteride or dutasteride.

Side Effects (Hepatotoxicity):

Mesterolone is not a C-17α alkylated product and is generally not considered hepatotoxic, making liver toxicity unlikely.

Side Effects (Cardiovascular):

As a synthetic/androgenic steroid, mesterolone may adversely affect serum cholesterol. This includes lowering HDL (good) cholesterol values and increasing LDL (bad) cholesterol values, potentially shifting the HDL to LDL balance and increasing the risk of arteriosclerosis. The impact of synthetic/androgenic steroids on blood lipids is based on the dosage, method of administration (oral vs. injectable), type of steroid (aromatizable or non-aromatizable), and level of resistance to hepatic metabolism. Mesterolone, as an orally administered non-aromatizable androgen, is generally considered to have a significant adverse effect on blood lipids. Studies have shown that men with hypogonadism taking 100 mg of mesterolone daily experienced a significant increase in total cholesterol (18.8%) after six months, with LDL cholesterol increasing by 65.2% and HDL decreasing by 35.7%.

Patients at risk for cardiovascular factors should not use mesterolone, especially when taking other oral steroids.

To help reduce cardiovascular strain, it is recommended to maintain an active cardiovascular exercise regime and always minimize the intake of saturated fats, cholesterol, and simple carbohydrates during AAS use. Supplementing with fish oil (4 grams per day) and natural cholesterol/antioxidant formulations like Lipid Stabil or similar products is also advisable.

Side Effects (Testosterone Suppression):

Mesterolone has very weak suppressive effects on gonadotropins and serum impact. Studies show that when subjects took a moderate dose (150 mg/day or less), testosterone levels were not significantly suppressed. When subjects took high doses (300 mg/day or higher), the drug strongly suppressed serum testosterone.

Therapeutic Guidance (Men):

The usual dose for treating androgen deficiency is: one 25 mg tablet three times daily at the start of therapy; maintain a lower dose of one 25 mg tablet once or twice daily thereafter. This dosage can also be used to aid male fertility, usually in combination with other fertility drugs, such as injectable FSH. Male athletes typically use a dose of 50-150 mg per day, or 2-6 tablets of mesterolone (25 mg each). The drug cycle usually lasts 6-12 weeks, which is long enough for therapeutic use.

When fitness athletes are in a dieting or competition preparation phase, they particularly desire a steroid with low estrogenicity and high anabolic rate, thus they favor mesterolone. It is especially beneficial when used alone with nandrolone, oxandrolone, or methenolone, as they have relatively low estrogenicity. Using mesterolone can also effectively adjust the ratio of androgens to estrogens, helping to increase muscle hardness and density, enhance libido, and promote fat burning. When used with other aromatizable steroids, it is commonly used to prevent gynecomastia, such as taking 10-20 mg of tamoxifen daily.

Therapeutic Guidance (Women):

Mesterolone is not approved for use in female users. Due to its strong androgenic properties and masculinizing side effects, it is not recommended for enhancing physique and performance in women. However, some women opt to take a 25 mg tablet, which can effectively alter their hormone levels and significantly affect body muscle lines. To minimize masculinizing side effects, the duration of use is usually no longer than 4-5 weeks. If an additional 10 or 20 mg of tamoxifen is taken, it can greatly increase muscle hardness and burn excess fat, especially in areas troubling for women such as buttocks and thighs. However, this usage should be approached with extreme caution.

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