In contrast to oral administration, the bioavailability of estradiol valerate is complete (i.e., 100%) via intramuscular injection.[5][3][4] Due to the far greater bioavailability of intramuscular estradiol valerate relative to oral, the former is substantially stronger (in terms of potency) than the latter.[3] 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.[3] 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.[3][117] 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.[4] In addition, a secondary depot may also be formed in adipose tissue.[4] 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.[3] 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.[8] 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.[3][118][119]