Chemical Formula: C26H40O3 Description Testosterone enanthate is an esterified variant of testosterone that comes as an injectable compound with a slow release rate. It is the first injectable ester preparation of testosterone. This slow release is achieved by the presence of the enanthate ester functional group attached to the testosterone molecule. This testosterone derivative was first approved on December 24, 1953. In 2017, about 6.5 million retail prescriptions for testosterone therapy were filled. The majority of the prescriptions written were for injectable (66%) and topical (32%) testosterone products. As recent as 1 October 2018, the US FDA approved Antares Pharma Inc.’s Xyosted – a subcutaneous testosterone enanthate product for once-weekly, at-home self-administration with an easy-to-use, single dose, disposable autoinjector. As the first subcutaneous autoinjector product designed for testosterone replacement therapy, this innovative formulation removes transfer concerns commonly associated with testosterone gels and potentially reduces the need for in-office/in-clinic injection procedures that may inconvenience patients with frequent visits to the clinic. Mechanism of action The effects of testosterone in humans and other vertebrates occur by way of two main mechanisms: by activation of the androgen receptor (directly or as DHT), and by conversion to estradiol and activation of certain estrogen receptors. Free testosterone (T) is transported into the cytoplasm of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5α-dihydrotestosterone (DHT) by the cytoplasmic enzyme 5α-reductase. DHT binds to the same androgen receptor even more strongly than T, so that its androgenic potency is about 2.5 times that of T. The T-receptor or DHT-receptor complex undergoes a structural change that allows it to move into the cell nucleus and bind directly to specific nucleotide sequences of the chromosomal DNA. The areas of binding are called hormone response elements (HREs), and influence transcriptional activity of certain genes, producing androgen effects. Such activities are useful as endogenous androgens like testosterone and dihydrotestosterone are responsible for the normal growth and development of the male sex organs and for maintenance of secondary sex characteristics. These effects include the growth and maturation of the prostate, seminal vesicles, penis, and scrotum; the development of male hair distribution, such as facial, pubic, chest, and axillary hair; laryngeal enlargement, vocal cord thickening, and alterations in body musculature and fat distribution. Male hypogonadism, a clinical syndrome resulting from insufficient secretion of testosterone, has two main etiologies. Primary hypogonadism is caused by defects of the gonads, such as Klinefelter’s syndrome or Leydig cell aplasia, whereas secondary hypogonadism is the failure of the hypothalamus (or pituitary) to produce sufficient gonadotropins (FSH, LH). Side effects Nausea, vomiting, headache, skin color changes, increased/decreased sexual interest, oily skin, hair loss, and acne may occur. Pain and redness at the injection site may also occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects when it is used at normal doses. Tell your doctor right away if you have any serious side effects, including: mental/mood changes (such as anxiety, depression, increased anger), trouble sleeping/snoring, signs of serious liver disease (such as persistent abdominal pain/nausea, unusual tiredness, yellowing eyes/skin, dark urine), hands/ankles/feet swelling, unusual tiredness, fast/irregular heartbeat. Get medical help right away if you have any very serious side effects, including: shortness of breath/rapid breathing, chest/jaw/left arm pain, unusual sweating, confusion, sudden dizziness/fainting, pain/swelling/warmth in the groin/calf, sudden/severe headaches, trouble speaking, weakness on one side of the body, sudden vision changes.