# Why don't flu shots leak?

**URL:** <https://boards.straightdope.com/t/why-dont-flu-shots-leak/327673>\
**Category:** Factual Questions\
**Created:** [October 24, 2005, 3:59pm UTC](https://boards.straightdope.com/t/why-dont-flu-shots-leak/327673 "2005-10-24T15:59:12Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![FlippyFly](https://avatars.discourse-cdn.com/v4/letter/f/e95f7d/32.png) [@FlippyFly](https://boards.straightdope.com/u/FlippyFly)\
**Post date:** [October 24, 2005, 3:59pm UTC](https://boards.straightdope.com/t/why-dont-flu-shots-leak/327673/1 "2005-10-24T15:59:12Z")

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I got a flu shot today. I could feel the liquid shooting into my arm. Why doesn’t it come back out? I’ve had acne with a lot less liquid that, well to be graphic, have shot out all over the place. Why doesn’t the flu shot leak?

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**Author:** ![St.Urho](https://avatars.discourse-cdn.com/v4/letter/s/8e7dd6/32.png) [@St.Urho](https://boards.straightdope.com/u/St.Urho)\
**Post date:** [October 24, 2005, 9:46pm UTC](https://boards.straightdope.com/t/why-dont-flu-shots-leak/327673/2 "2005-10-24T21:46:45Z")

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I believe flu shots are given intramuscularly (IM). There are two techniques for preventing the medication from leaking back out during IM administration- the air lock and the z-track. The air lock involves drawing up a small amount of air (~1mL) and injecting the med so that the air goes in after the med. The z-track involves moving the skin so that after the injection, the needle track in the subcutaneous layer does not line up with the injection site in the muscle. See the illustration on [this](http://www.nurseminerva.co.uk/giving.htm) site.

St. Urho  
Paramedic

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**Author:** ![picunurse](https://avatars.discourse-cdn.com/v4/letter/p/7bcc69/32.png) [@picunurse](https://boards.straightdope.com/u/picunurse)\
**Post date:** [October 25, 2005, 2:55am UTC](https://boards.straightdope.com/t/why-dont-flu-shots-leak/327673/3 "2005-10-25T02:55:26Z")

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**St. Urho** is correct, however, without either method, most IM injections don’t leak anyway. The medication is forced between the muscle fibers. There really isn’t a pressure gradient to push the liquid out. Its a small enough amount that its gets absorbed into the tissue quickly.

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**Author:** ![FilmGeek](https://avatars.discourse-cdn.com/v4/letter/f/b19c9b/32.png) [@FilmGeek](https://boards.straightdope.com/u/FilmGeek)\
**Post date:** [October 25, 2005, 3:01am UTC](https://boards.straightdope.com/t/why-dont-flu-shots-leak/327673/4 "2005-10-25T03:01:38Z")

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I used to get depo-provera shots and they leaked like crazy. Aren’t those intramuscular?