What is the preferred site for intramuscular (IM) vitamin B12 injections?

Preferred Site for Intramuscular Vitamin B12 Injections

For adults, use the deltoid muscle as the preferred site for intramuscular vitamin B12 injections, and for infants and young children, use the anterolateral thigh. 1, 2, 3

Site Selection by Age Group

Adults and Older Children

• The deltoid muscle is the CDC-recommended site for routine intramuscular injections in adults, including vitamin B12 administration 1, 2, 4, 3
• Use a 1 to 1½ inch, 22-25 gauge needle to ensure proper muscle penetration 2, 4, 3
• The optimal injection point is at the intersection between the anteroposterior axillary line and the perpendicular line from the mid-acromion, which provides maximum distance from the axillary nerve and posterior circumflex humeral artery 5
• A practical landmark is 4-5 fingerbreadths (approximately 9-10 cm) below the mid-acromion point, which avoids underlying neurovascular structures 6, 7

Infants and Young Children

• The anterolateral aspect of the thigh is the preferred site for infants and young children 1, 2, 3
• Use a 7/8 to 1¼ inch, 22-25 gauge needle based on muscle size 2, 3
• For toddlers and older children with adequate deltoid muscle mass, the deltoid can be used 2, 3

Critical Sites to Avoid

Gluteal Region

• The CDC explicitly advises against using the buttock for routine intramuscular injections due to risk of sciatic nerve injury 2, 4, 3
• Gluteal injections are associated with decreased immunogenicity of certain medications, likely due to inadvertent subcutaneous or deep fat tissue injection rather than true intramuscular administration 2

Proper Injection Technique

Needle Insertion

• Insert the needle at a 90-degree angle for all intramuscular injections 2, 3
• The needle must be long enough to reach muscle mass but not so long as to involve underlying nerves, blood vessels, or bone 2, 3
• Depth of insertion should be at least 5 mm greater than subcutaneous thickness to ensure intramuscular delivery 5

Common Pitfalls

• Inadequate needle length results in subcutaneous rather than intramuscular administration, potentially reducing medication efficacy 2
• Injecting too high on the deltoid (in the upper half) risks axillary nerve injury—always inject at or below the midpoint of the deltoid muscle 6, 7
• The middle of the rectus femoris should be avoided due to the presence of major blood vessels; if using the thigh in adults, the middle of the vastus lateralis is safer 8

Pharmacokinetic Considerations for Vitamin B12

• Cyanocobalamin is quantitatively and rapidly absorbed from intramuscular injection sites, with plasma levels peaking within 1 hour 9
• Intramuscular administration allows for adequate liver storage, unlike intravenous administration which results in rapid urinary excretion 9
• The intramuscular route is essential for patients with pernicious anemia or malabsorption conditions where oral absorption is unreliable 9