Intramuscular (IM) Injection
Also known as: IM · intramuscular injection
An intramuscular (IM) injection delivers a drug into a muscle, using a longer needle than a subcutaneous injection, at sites such as the deltoid.
In an intramuscular injection the needle passes through the skin and the fat layer and into muscle. Common adult sites include the deltoid in the upper arm and the gluteal region, where the ventrogluteal site on the side of the hip is one option. Because the needle has to cross the fat layer first, needle length has to suit the person: in a CT study of deltoid fat thickness, a 1-inch needle was expected to reach the muscle in 85.3% of women under 200 pounds and 98.6% of men under 260 pounds, but only in about three quarters of heavier women and men. The same study found age and BMI predicted fat thickness better than weight, and concluded that weight-based needle guidelines leave women and people with obesity more likely to get a subcutaneous injection when an intramuscular one was intended.
Route changes how a drug is absorbed. For insulin, injection-technique guidance says intramuscular injections should be avoided, especially with long-acting insulins, because severe low blood sugar can result.
Why it matters
For any peptide with an approved label, the route on that label is the one that was tested. Switching a product meant for SubQ use to IM, or reaching muscle by accident with a long needle, can change the speed and size of its effect; that is the reason insulin guidance warns against IM injection.
If you do give an IM injection, technique affects comfort. A systematic review found that injecting into the ventrogluteal site hurt less than the dorsogluteal site, and that manual pressure or rhythmic tapping at the site reduced pain, although the studies were small and varied. The same review found too little evidence that the Z-track or cold-needle techniques reduce pain, and found that warming the injected liquid did not.
How to approach it
- Check the route on the product's label or instructions; if it says subcutaneous, do not inject it into muscle.
- If intramuscular is the stated route, choose a needle long enough to cross the fat layer at that site, which varies with sex, age and BMI.
- For gluteal injections, the ventrogluteal site was less painful than the dorsogluteal site in pooled studies.
- Applying manual pressure to the site was linked to less injection pain.
Common mistakes
- Switching a product labeled for subcutaneous use to intramuscular injection on your own.
- Using a short insulin needle and assuming it reached muscle.
- Using a needle too short for the person's body size, which can leave the dose in fat instead of muscle.
- Assuming IM and SubQ give the same absorption and effect.
- Using the same IM site repeatedly without rotating.
Sources
- ReferenceStatistical estimation of deltoid subcutaneous fat pad thickness: implications for needle length for vaccination. Sci Rep, 2022. PMID 35058499.
- GuidelineNew Insulin Delivery Recommendations. Mayo Clin Proc, 2016. PMID 27594187.
- Systematic reviewThe effect of intramuscular injection technique on injection associated pain; a systematic review and meta-analysis. PLoS One, 2021. PMID 33939726.
Page updated . How we grade evidence: methodology.
Related Terms
Subcutaneous (SubQ) Injection
A subcutaneous (SubQ) injection delivers a drug into the fat layer between the skin and the muscle, where it is absorbed gradually.
Injection Site Rotation
Injection site rotation means moving each injection to a different spot so the same patch of tissue is not injected repeatedly.
Insulin Syringe
An insulin syringe is a small, fine-needle syringe marked in insulin units; on the common U-100 type, 100 units equals 1 mL.
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