Why Are Some Shots Given in the Arm and Some in the Bum?

When you walk into a doctor's office to get a shot, sometimes it's a surprise. Most of the time you can count on getting an injection in your arm, but occasionally they'll pitch you a curveball and inject it into your rear end or even your belly. Why is this?

Roll Up Your Sleeve or Pull Down Your Pants

It generally has to do with the type of medicine being injected, the volume of medicine, and how quickly or slowly the medicine needs to be absorbed into the body.

There are several different ways to get medicine into your body with a needle:

Intravenous injections go directly into the vein and deliver medicine really quickly — they are normally delivered in the back of the hand or the front of the elbow.
Intramuscular shots are injected directly into a muscle, where the medicine is absorbed a little more slowly by blood vessels — the most common locations for these are the deltoid muscle of the shoulder or arm and the gluteus medius of the bum, or in the thigh muscle, or vastus lateralis, for little children.
Subcutaneous injections are given into fatty tissue, where there is less blood supply, and therefore the medicine is taken up by the body more slowly — these are generally injected into the abdominal fold.
Intradermal injections target the middle layer of the skin and are absorbed most slowly of all. Common injection sites for these include the inner surface of the forearm and the upper back, under the shoulder blade.

Not All Shots Are Created Equal

"The injection site is driven by how the medication is absorbed," says Libby Richards, a faculty associate at the Purdue University School of Nursing. "Some medications, like insulin, needs to be absorbed slowly so fatty tissue without as much blood flow works better. Medications injected into the muscles are absorbed faster than fatty tissues but slower than intravenous."

It's common for antibiotics, diuretics and analgesics to be administered intravenously, for instance, while many vaccines, hormone shots and allergy medications are injected intramuscularly.

In addition to the type of medicine, doctors and nurses need to think about how much medicine needs to be injected, and whether a specific muscle is large enough to hold that much medication.

"Muscle tissue can generally hold more volume than fatty, or subcutaneous, tissue so that is another consideration when choosing an injection site," says Richards. "When the volume of medication is a consideration, larger muscles such as the buttocks or thigh may be used instead of the arm. Plus, some medications can cause irritation to delicate blood vessels — in this case, muscle is preferred as well."

And even with all the considerations, you can still ask your doctor if they can give your injection in your preferred location:

"Sometimes it comes down to patient preference and convenience," says Richards. "The arm is often easier to access and preferred by patients."