Fluoxymesterone, sold under the brand names Halotestin and Ultandren among others, is an androgen and anabolic steroid (AAS) medication which is used in the treatment of low testosterone levels in men, delayed puberty in boys, breast cancer in women, and anemia.

[1][12] It is specifically approved in one or more countries for the treatment of hypogonadism in men, delayed puberty in boys, and breast cancer in women.

[13] Current prescribing guidelines in the United States list only the treatment of androgen deficiency in males and breast cancer in females as indications.

[1][17] It is a substrate for 5α-reductase like testosterone, and so is potentiated in so-called "androgenic" tissues like the skin, hair follicles, and prostate gland via transformation into 5α-dihydrofluoxymesterone.

[10] Fluoxymesterone has been reported to be non-aromatizable due to steric hindrance by its C11β hydroxyl group,[18] and hence is not considered to have a propensity for producing estrogenic effects such as gynecomastia or fluid retention.

The regioselectivity of pyrrolidine for reaction at the 3α-position occurs inherently in the structure of adrenosterone, due to the position of the sterically bulky methyl groups.

Step two: The 11α-hydroxyl of the starting material (1.16) is sulfonylated by p-toluenesulfonyl chloride; addition of trimethylamine (base) deprotonates the 11α-carbon, yielding an (E2) elimination of tosylate (pka - 5) to give olefin (1.17).

The hydroxide ion then attacks from above the ring (β-face) at the 11-carbon, resulting in a structure (1.18) by the stereospecific addition of hydroxyl and bromine across the double bond.

In a concerted process, fluoride attacks the ring's α-face from below, as one of the two oxygen-carbon bonds is broken on the opposite face; hence regenerating the 11α-hydroxyl trans to the fluorine substituent.

In a test for halotestin, a dry residue obtained from a urine sample is dissolved in dimethylformamide and a sulfur trioxide-pyridine complex and is heated with 1% potassium carbonate solution.

Halotestin and many of its metabolites contain two polar hydroxyl groups, leading to intermolecular hydrogen bonding that increases their boiling point and reduces volatility.

In order to attain a gaseous sample for GC-MS, the products of hydrolysis are extracted, dissolved in methanol and derivatised to form volatile trimethylsilyl (TMS) esters by adding N-methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA) and trimethylsilylimidazole (TMSImi).