An intramuscular (IM) injection delivers medication deep into muscle tissue, where a rich blood supply absorbs it quickly. The needle goes in at a 90-degree angle to the skin, straight into the muscle belly. Whether you’re giving a vaccine, a prescribed medication like epinephrine, or a hormone injection at home, the core technique is the same: choose the right site, use the right needle, insert at 90 degrees, inject steadily, and withdraw.
Choosing the Right Injection Site
Three muscles are commonly used for IM injections, and each one suits different situations. The site you choose depends on the volume of medication, the patient’s age, and how much muscle mass is available.
Deltoid (upper arm): This is the most common site for vaccines in adults and children over age 3. To find it, locate the bony point at the top of the shoulder (the acromion process). The injection site sits roughly 2 inches below that bone and above the armpit fold. The deltoid is convenient but relatively small, so it holds a maximum of about 2 mL of fluid.
Vastus lateralis (outer thigh): This is the preferred site for infants and young children because the thigh has more muscle mass than their small deltoid. In adults, the outer middle third of the thigh works well for self-injection since it’s easy to see and reach. The thigh can handle up to 5 mL in adults and up to 1 mL in newborns.
Ventrogluteal (hip): Located on the side of the hip, this site is well-suited for larger volumes (up to 3 mL) and medications that may irritate tissue. It’s considered one of the safest sites because it sits far from major nerves and blood vessels, though it requires a second person to administer.
Selecting the Right Needle
The needle needs to be long enough to pass through skin and fat and actually reach the muscle. Too short, and the medication ends up in the fat layer, where absorption is poor and irritation is more likely. CDC guidelines recommend a 22- to 25-gauge needle for IM injections across all age groups, with length varying by body size.
- Infants (1 to 11 months): 1-inch needle, into the thigh
- Children (1 to 10 years): 1 to 1.25 inches for the thigh; ⅝ to 1 inch for the deltoid
- Adults under 130 lbs: 1-inch needle into the deltoid
- Adults 130 to 200 lbs (women) or 130 to 260 lbs (men): 1 to 1.5 inches into the deltoid
- Adults over 200 lbs (women) or over 260 lbs (men): 1.5-inch needle into the deltoid
For adults who weigh under 130 lbs, some clinicians use a ⅝-inch needle, but only if the skin is stretched tightly and the underlying fat is not bunched up during insertion.
Supplies You’ll Need
Gather everything before you start so you’re not reaching for items mid-procedure. You’ll need the prefilled syringe or vial with the correct needle attached, an alcohol swab, clean gloves, a small piece of gauze, an adhesive bandage, and a sharps disposal container. Having everything within arm’s reach makes the process faster and smoother, which matters for keeping both you and the patient calm.
Step-by-Step Injection Technique
Wash your hands thoroughly and put on gloves. Draw up the medication if it isn’t prefilled, and remove any air bubbles by tapping the syringe and pushing the plunger gently until a small drop appears at the needle tip.
Clean the injection site with an alcohol swab using a firm circular motion, then let the skin air dry completely. Injecting through wet alcohol can sting and may carry surface bacteria into the tissue.
With your non-dominant hand, stretch the skin taut over the injection site. This anchors the tissue and helps the needle pass through more easily. Hold the syringe like a dart in your dominant hand and insert the needle at a 90-degree angle in one smooth, quick motion. Hesitating or going slowly increases pain.
Push the plunger down with steady, even pressure. Inject the medication over a few seconds rather than all at once. Once the syringe is empty, wait about 10 seconds before pulling the needle out. This brief pause allows the medication to begin dispersing in the muscle and reduces the chance of it leaking back through the needle track.
Withdraw the needle at the same angle it went in, activate the safety device if your needle has one, and immediately dispose of it in a sharps container. Press a piece of gauze gently over the site and apply a bandage.
Should You Aspirate?
Aspiration means pulling back on the plunger after inserting the needle to check for blood. For decades, this was standard practice. Current CDC guidelines say aspiration is not necessary for vaccine injections because no large blood vessels exist at the recommended injection sites. Skipping aspiration also makes the process less painful, particularly for infants. For non-vaccine medications, follow the specific instructions that come with the drug, as some still call for aspiration.
The Z-Track Method
Some medications, particularly those that stain or irritate the tissue under the skin, call for a modified technique called the Z-track method. Instead of simply stretching the skin taut, you pull it about half an inch (1 cm) to the side before inserting the needle. Hold it displaced throughout the injection. After withdrawing the needle, release the skin so it slides back into its natural position. This creates a zigzag path that seals the medication inside the muscle and prevents it from leaking back up into the tissue closer to the surface.
When using the Z-track method, do not massage the site afterward. Rubbing can push the medication out of the muscle. Avoid tight clothing over the area for the same reason.
Reducing Pain During Injection
A few simple techniques make a noticeable difference in comfort. Stretching the skin firmly before insertion reduces the sensation of the needle breaking through. Injecting at a steady pace rather than pushing the plunger too fast or too slow also helps. For children aged 5 to 10, applying vibration or gentle pressure near the site during the injection has been shown to significantly reduce pain, anxiety, and fear. Devices like the Buzzy (a small vibrating device with a cold pack) work on the same principle, essentially distracting the nearby nerves so they register less pain.
A newer technique called TPR (traction, pressure, and rapid muscle release) has shown promise in clinical trials. It involves applying traction and deep pressure to the muscle, inserting the needle, then quickly releasing the muscle toward the needle. In one randomized trial, patients rated their pain at roughly 1.7 out of 10 with this method compared to 3.8 with the standard Z-track approach.
What to Watch for Afterward
Some soreness, mild swelling, or redness at the injection site is normal and typically fades within a day or two. Applying a cool compress can help with discomfort. Avoid soaking the site in a bath for 24 hours, though showering is fine.
Contact a healthcare provider if you notice signs of infection: a fever over 100.4°F on two readings taken four hours apart, increasing redness or swelling that spreads from the site, or any drainage.
Potential Complications
Serious complications from properly placed IM injections are rare, but they do happen. The most significant risk is nerve injury, which occurs when the needle is inserted too close to a nerve bundle. In a large surgical case series of 354 patients treated for injection-related nerve damage, the sciatic nerve in the buttock was involved in nearly 43% of cases, and the radial nerve in the upper arm accounted for another 37%. Symptoms include immediate sharp or burning pain radiating down the limb, numbness, tingling, or weakness.
This is precisely why the dorsogluteal site (the upper outer buttock) has fallen out of favor for most injections. The ventrogluteal site on the hip, the deltoid, and the outer thigh all carry a much lower risk of hitting a major nerve. Choosing the correct site and using proper landmarks is the single most important thing you can do to avoid this kind of injury.