Thank you for your inquiry.
Three routes have been studied regarding the administration of epinephrine subcutaneously and intramuscularly. The routes of administration are:
Intramuscular injection in the upper arm (deltoid muscle).
Subcutaneous injection in the upper arm.
Intramuscular injection in the lateral thigh (vastus lateralis muscle).
The results of these studies (1, 2) demonstrate that intramuscular injection in the thigh (but not the upper arm) results in the fastest rise of blood levels of epinephrine. Intramuscular injection in the upper arm (deltoid) and subcutaneously in the upper arm result in a much slower absorption.
Because of the far more rapid rise in blood level due to intramuscular injection in the lateral thigh, the authors of these articles have recommended that this is the preferred route of administration of epinephrine. The rationale for this conclusion is based on the fact that severe anaphylactic reactions can be rapidly fatal and therefore the faster the rise in blood levels, the more potentially effective the injection will be in preventing death.
Thus the issue per se is not intramuscular versus subcutaneous injections, but intramuscular injection in the thigh versus intramuscular injection in the deltoid muscle and subcutaneous injection in the arm.
Thank you again for your inquiry and we hope this response is helpful to you.
Simons FER, et al. Epinephrine absorption in children with a history of anaphylaxis. J Allergy Clin Immunology 1998; 101(1):33-37.
Simons FER, et al. Epinephrine absorption in adults: intramuscular versus subcutaneous injection. J Allergy Clin Immunol 2001; 108(5):871-873.
Phil Lieberman, M.D.