the former is substantially stronger (in terms of potency) than the latter. As an example, a single 4 mg intramuscular injection is said to be approximately equivalent to 2 mg/day of the medication administered orally over the course of 3 weeks. Estradiol valerate, when given intramuscularly in oil, has a relatively long duration due to the formation of an intramuscular depot from which the medication is slowly released and absorbed. Upon intramuscular injection of estradiol valerate in an oil solution, the solvent (i.e., oil) is absorbed, and a primary microcrystalline depot is formed within the muscle at the site of injection. In addition, a secondary depot may also be formed in adipose tissue. The slow release of estradiol valerate is caused by the increased lipophilicity of the medication, which in turn is due to its long fatty acid valeric acid ester moiety. The elimination half-life of estradiol valerate in oil by intramuscular injection (brand names Estradiol-Depot 10 mg, Progynon Depot-10) is about 3.5 days, with a range of 1.2 days to 7.2 days in different individuals. Α couple of older studies from the 1980s with sample sizes of only 2 or 3 individuals reported an elimination half-life of 4 to 5 days. A single intramuscular injection of 4 mg estradiol valerate has been found to result in maximal circulating levels of estradiol of about 390 pg/mL within 3 days of administration, with levels declining to 100 pg/mL (baseline, in the study) by 12 to 13 days. Studies in general have found that a single intramuscular injection of 4 mg estradiol valerate results in peak levels of estradiol of 240 to 540 pg/mL after 1 to 5 days following administration. A study found that a single intramuscular injection of 5 mg estradiol valerate resulted in peak circulating levels of 667 pg/mL estradiol and 324 pg/mL estrone within approximately 2 and 3 days, respectively. The duration of estradiol valerate at this dose and in this study was considered to be 7 to 8 days. Other studies have found that larger doses of intramuscular estradiol valerate exceeding 20 mg have a duration of more than 15 days. A third study, in contrast to the preceding study, found that a single 10 mg intramuscular injection of estradiol valerate resulted in maximal estradiol levels of 506 to 544 pg/mL and maximal estrone levels of 205 to 219 pg/mL in postmenopausal women. With intramuscular injections of