There are only 3 sites for intramuscular injections, most commonly the deltoid then the ventrogluteal, and lastly the vastus lateralis for infants. If the site is a "hard muscle" or when the muscle is in constant spasm, then an intramuscular injection should not be done. Other reasons why an intramuscular injection cannot be done on a specific site are (1) atrophied muscle on an emanciated patient (i.e. malnourished or cachectic patients), (2) skin infection overlying the muscle group, and (3) the dorsogluteal muscle as this can hit the sciatic nerve and cause paralysis and other neuropathy of the ipsilateral leg.
