But... why the thigh? Why not the arm like most other shots?
Per AAAAI, there have been 3 anatomic sites investigated to determine which location offers the fastest systemic absorption of epinephrine when injected intramuscularly and/or subcutaneously.
- Intramuscular injection in the upper arm (deltoid muscle) just like a flu shot.
- Subcutaneous injection in the upper arm just like an allergy shot.
- Intramuscular injection in the lateral thigh (vastus lateralis muscle).
When dealing with a life-threatening anaphylaxis, you need the speed because if the drug is too slow, the person may die before adequate epinephrine levels are achieved... and the thigh muscle provides the speed whereas the other sites do not.
Why might that be?
It's probably because the thigh muscle is large with lots of blood vessels allowing for multiple points of access for the injected epinephrine to enter into the blood circulation.
Epinephrine absorption in children with a history of anaphylaxis. J Allergy Clin Immunology 1998; 101(1):33-37.
Epinephrine absorption in adults: intramuscular versus subcutaneous injection. J Allergy Clin Immunol 2001; 108(5):871-873.