Depot Injection: How Slow-Release Shots Work
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[Glossary · How doses act in the body]

Depot injection

Also called depot, depot formulation, long-acting injection, depot effect · Reviewed

Definition: A depot is a store of drug left at the injection site that releases into the blood slowly over days or weeks. A depot injection is made to do this on purpose, for example by dissolving a drug in oil so it leaves the site slowly. Testosterone cypionate in oil is a common example.

How a depot works: the testosterone example

The Depo-Testosterone label explains it plainly. Testosterone esters are less polar than free testosterone. Injected into muscle in oil, they are absorbed slowly from the oil, so testosterone cypionate can be given at intervals of two to four weeks.

Each mL of the 100 mg/mL product contains testosterone cypionate 100 mg in cottonseed oil (736 mg), with benzyl benzoate and benzyl alcohol as a preservative. The label gives the half-life of testosterone cypionate injected into muscle as about eight days.

Worked example: what the slow release does

An 8-day half-life over a 2-week interval

  1. Using the label half-life of about 8 days, about 50% of one shot is left after 8 days.
  2. At 16 days, two half-lives: 50% × 0.5 = 25% left.
  3. So a dose given every 14 days still has a good share left when the next one goes in, and levels build toward steady state.

This uses the simple model where each half-life halves what is left. A depot's release rate depends on the oil, the site and the person.

Common mix-ups

  • Not every long half-life is a depot. The Ozempic label gives semaglutide a half-life of about 1 week and says it is more than 99% bound to albumin in the blood. It does not describe the shot as a depot.
  • The depot sets the interval. For a testosterone ester, the Depo-Testosterone label ties the two to four week interval to slow absorption from the oil.

Read more on Dose Track: Testosterone cypionate guide · Half-life calculator

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
Testosterone esterA testosterone ester is testosterone with a fatty acid chain attached, such as cypionate or enanthate. The… Intramuscular injection (IM)An intramuscular injection (IM) puts medicine deep into a muscle, below the fat layer. Most testosterone… Half-lifeHalf-life is the time it takes for the amount of a drug in your body to fall by half. After one half-life,… Steady stateSteady state is the point, after repeated doses on a fixed schedule, where the amount cleared between…

Frequently asked questions

What does depot mean in medicine?+
It means a store of drug left at the injection site that releases slowly into the blood, so one shot lasts days or weeks.
Why is testosterone cypionate a depot?+
The Depo-Testosterone label says the ester in oil is absorbed slowly from the oil after an injection into muscle, which is why it can be given every two to four weeks.

Sources

  1. Depo-Testosterone (testosterone cypionate) US prescribing information, Description and Clinical Pharmacology, DailyMed
  2. Ozempic (semaglutide) US prescribing information, sections 2 and 12.3, DailyMed

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