Intramuscular Injection Technique
Intramuscular injections must be administered at a 90-degree angle to the skin surface to ensure proper muscle penetration and optimal drug absorption. 1
Injection Angle
The standard angle for IM injections is 90 degrees perpendicular to the skin surface, as recommended by the Advisory Committee on Immunization Practices (ACIP) and the American Academy of Family Physicians. 1
This 90-degree angle is critical because deviating from the recommended route can reduce medication efficacy or increase local adverse reactions. 1
Research confirms that a 90-degree angle is most effective for patient comfort, safety, and medication efficacy compared to other angles. 2
While one study suggests angles between 72-90 degrees may be acceptable (as 72 degrees reaches 95% of the depth of 90 degrees), the guideline-recommended standard remains 90 degrees. 3
Recommended Needle Specifications by Age
Infants (<12 months)
- Needle size: 7/8 to 1 inch length, 22-25 gauge 1
- Injection site: Anterolateral aspect of the thigh (provides the largest muscle mass in infants) 1
Toddlers and Children (12 months to 18 years)
- Needle size: 7/8 to 1¼ inches length, 22-25 gauge 1
- Injection site: Deltoid muscle if adequate muscle mass is present; otherwise anterolateral thigh 1, 4
- For toddlers using the thigh, the needle should typically be 1 inch long. 1
Adults (>18 years)
- Needle size: 1 to 1½ inches length, 22-25 gauge 1
- Injection site: Deltoid muscle (preferred) or anterolateral thigh 1, 4
Critical Site Selection Guidelines
Never use the buttock for IM injections due to the risk of sciatic nerve injury and decreased immunogenicity from inadvertent subcutaneous or deep fat tissue injection. 1, 4, 5
The needle must be long enough to reach muscle mass and prevent medication from seeping into subcutaneous tissue, but not so long as to involve underlying nerves, blood vessels, or bone. 1
Important Technical Considerations
Needle length selection must account for patient body habitus: In obese patients (BMI >24.9 kg/m²), standard needles may not reach muscle, particularly in women where subcutaneous tissue can exceed 40-50 mm at common injection sites. 6
Aspiration is not necessary: Although some practitioners advocate for aspiration before injection, no data support this as a required procedure. If blood appears in the needle hub during aspiration, withdraw and select a new site. 1
When administering multiple IM injections in the same limb, separate injection sites by more than 1 inch to differentiate any local reactions. 1
Common Pitfalls to Avoid
Using inadequate needle length for patient body size, which results in subcutaneous rather than intramuscular delivery and reduced medication efficacy. 1, 6
Injecting into the buttock, which carries both safety risks (sciatic nerve injury) and efficacy concerns (decreased drug absorption). 1, 4, 5
Failing to individualize needle selection based on age, muscle mass, and injection volume, as a one-size-fits-all approach increases the risk of improper administration. 1