It is administered at intervals of once every 1 to 2 weeks in men. Microcrystalline testosterone isobutyrate in aqueous suspension requires a larger needle (21 gauge) than oil solutions due to the presence of solid crystals in the suspension. It can produce local irritation, pain, and redness upon injection. Testosterone isobutyrate, or testosterone 17β-(2-methylpropanoate), is a synthetic androstane steroid and a derivative of testosterone. It is an androgen ester; specifically, it is the C17β 2-methylpropanoate (isobutyrate) ester of testosterone. Microcrystalline testosterone isobutyrate in aqueous suspension was first described in 1952. It was introduced for medical use shortly thereafter. Around the same time, testosterone enanthate in oil solution was introduced for medical use. It became the dominant long-acting injectable form of testosterone, and limited the commercial success of testosterone isobutyrate. The combination of microcrystalline estradiol benzoate and testosterone isobutyrate in aqueous suspension was introduced under the brand name Femandren M by 1953. Brand names of testosterone isobutyrate include Agovirin-Depot, Perandren M, Testocryst, and Virex-Cryst. It has also been marketed in combination with estradiol benzoate under the brand names Femandren M and Folivirin. Testosterone isobutyrate is available only in the Czech Republic and Slovakia. It was originally manufactured by the pharmaceutical company SPOFA intermittently manufactured by Biotika, and is now manufactured by BB Pharma.