system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is a single injection of a drug able to provide months of therapy?

My wife is getting a single shot of a drug that will stop her production of estrogen for a full three months. I read about "once a year" injections that can provide months of relief for various conditions. How can a single dose of a medication remain metabolically active for months at a time?
gpt
Esterification of an injectable steroid basically accomplishes one thing, it slows the release of the parent steroid from the site of injection. 
This happens because the ester will notably lower the water solubility of the steroid, and increase its lipid (fat) solubility. This will cause the drug to form a deposit in the muscle tissue, from which it will slowly enter into circulation as it is picked up in small quantities by the blood. Generally, the longer the ester chain, the lower the water solubility of the compound, and the longer it will take to for the full dosage to reach general circulation.