An intramuscular (IM) injection delivers medication directly into a large muscle, where a rich blood supply absorbs the drug quickly into your bloodstream. It’s one of the most common ways medications are given in clinical settings, from vaccines to antibiotics to hormone treatments. If you’ve ever gotten a flu shot in your upper arm, you’ve had an IM injection.
How IM Injections Work
The key advantage of injecting into muscle tissue is speed and reliability. Muscles have an extensive network of blood vessels, so medication deposited there enters systemic circulation quickly, typically faster than a shot given just under the skin (subcutaneous). An IM injection also bypasses what’s called first-pass metabolism, meaning the drug doesn’t have to pass through your digestive system and liver before reaching the rest of your body. That’s why certain medications that would be broken down or poorly absorbed if swallowed are given this way instead.
Compared to an intravenous (IV) line, an IM injection is far simpler. There’s no need to find a vein or keep a catheter in place. The tradeoff is that absorption isn’t quite as instant as IV delivery, but for most medications, the difference doesn’t matter clinically.
Where IM Injections Are Given
Not every muscle is a good candidate. The injection site needs to be large enough to absorb the medication, well supplied with blood, and located safely away from major nerves and blood vessels. Four sites are used most often:
- Deltoid (upper arm): The most familiar site for adults. The injection goes about 2.5 to 5 centimeters below the bony point at the top of your shoulder. This is where most vaccines are given.
- Vastus lateralis (outer thigh): The preferred site for infants and young children because their thigh muscles are more developed than their arm muscles at that age. The target is the middle third of the outer thigh.
- Ventrogluteal (hip): A site on the side of the hip, commonly used for adults when a larger volume of medication needs to be injected. It’s considered one of the safest sites because it has thick muscle and is far from major nerves.
- Dorsogluteal (upper outer buttock): The classic “shot in the buttock.” This site is used less frequently today because of its proximity to the sciatic nerve.
The recommended site changes with age. Infants typically receive IM injections in the outer thigh. Children can use either the thigh or the deltoid. Adults most commonly get injections in the deltoid or the ventrogluteal area.
What the Needle Looks Like
IM injections use a 22- to 25-gauge needle, which is relatively thin. The length varies based on your body size and which muscle is being targeted, because the needle has to pass through skin and any subcutaneous fat to actually reach the muscle.
For adults, CDC guidelines recommend a 1-inch needle for people weighing 130 pounds or less. At 130 to 152 pounds, either a 1-inch or 1.5-inch needle may be used. For men over 152 pounds and women over 152 pounds, a 1.5-inch needle is standard. For infants receiving a thigh injection, a 1-inch needle is typical, while newborns may need only a 5/8-inch needle.
Getting the length right matters. A needle that’s too short deposits the medication into fat rather than muscle, which slows absorption and can reduce the effectiveness of vaccines. A needle that’s too long risks hitting bone or deeper structures.
The Z-Track Technique
You may hear a nurse mention the “Z-track” method. Originally developed for iron injections that could stain or irritate the tissue under the skin, this technique is now recommended for all IM injections. The provider pulls the skin and tissue to one side before inserting the needle, gives the injection, and then releases the skin after withdrawing the needle. This creates a zigzag path through the tissue layers that seals itself, preventing medication from leaking back into the subcutaneous layer. It reduces irritation, minimizes pain, and helps ensure the full dose stays in the muscle where it belongs.
What You’ll Feel During and After
Most people feel a brief sting or pinch as the needle goes in, followed by a sensation of pressure as the medication is injected. The whole process takes only a few seconds. Soreness at the injection site is normal and can last one to three days, especially with vaccines. You might also notice mild swelling, redness, or warmth around the area.
One older practice, aspiration (pulling back on the syringe to check for blood before injecting), is no longer recommended by the World Health Organization or the CDC for routine IM injections. Studies have found that aspiration increases pain, particularly in children, without providing a meaningful safety benefit at the standard injection sites.
Common Medications Given by IM Injection
A wide range of medications use this route. Vaccines are the most common example: flu shots, COVID-19 vaccines, tetanus boosters, and most childhood immunizations are all given intramuscularly. Beyond vaccines, IM injections are used for certain antibiotics (particularly when a quick dose is needed and swallowing a pill isn’t practical), hormone therapies like testosterone and progesterone, vitamin B12 for people with absorption problems, epinephrine for severe allergic reactions, and some long-acting psychiatric medications designed to release slowly from the muscle over weeks.
The deltoid can typically hold up to about 1 to 2 milliliters of fluid, which is enough for most vaccines. When a larger volume is needed, providers choose the ventrogluteal or vastus lateralis sites, which can accommodate more.
Risks and Complications
Serious complications from IM injections are uncommon, but they do happen. The most significant risk is nerve injury. The sciatic nerve, which runs through the buttock and down the leg, is the nerve most frequently damaged by IM injections. This is a major reason the dorsogluteal (buttock) site has fallen out of favor for routine injections.
If a needle strikes or injects medication near the sciatic nerve, the sensation is unmistakable: an immediate electric shock or burning pain that shoots down the leg. Depending on severity, this can cause numbness, tingling, weakness, or in rare cases partial paralysis in the affected leg. Recovery varies widely. Some people improve within weeks, while others experience lasting pain or sensory changes. Using the ventrogluteal site instead of the dorsogluteal site dramatically lowers this risk.
Other possible complications include localized infection at the injection site, small areas of bruising, and, in rare cases, the formation of a hard lump (called a nodule) under the skin if medication is accidentally deposited in the fat layer instead of the muscle. Allergic reactions to the medication itself can also occur, though these are related to the drug rather than the injection technique.