Intramuscular (IM) injection

Intramuscular administration delivers a solution into a muscle, usually the deltoid, the thigh or the gluteal muscle, with a longer needle than a subcutaneous injection uses. Muscle is richly supplied with blood, so absorption is faster and peak levels arrive sooner than from subcutaneous fat, at the cost of more discomfort and a greater need for correct technique.

Few research peptides call for it. Most of the literature uses the subcutaneous route because it is simpler, better tolerated and gives a steadier profile. The exceptions tend to be larger molecules and clinical products with an IM tradition: HCG is given IM or SC in fertility practice, some cerebrolysin trials used IM dosing, and older thymosin alpha-1 regimens were IM. Each profile's "Research protocols" section states the route the cited studies used.

The comparison article on subcutaneous versus intramuscular injection sets out the differences in absorption, technique and risk. For research handling, the default remains subcutaneous unless a specific study's protocol is being reproduced.