all the information we have in our books points to the ventrogluteal site as the best site due to the fact that it is furthest from major nerves and blood vessels, problem is it's not always accessible. here's from my book:
• Ventrogluteal: The ventrogluteal muscle involves the gluteus medius and minimus. It is situated deep and away from major nerves and blood vessels and is a safe site for all clients. Research has shown that injuries such as fibrosis, nerve damage, abscess, tissue necrosis, muscle contraction, gangrene, and pain have been associated with all the common IM sites except the ventrogluteal site (Nicoll and Hesby, 2002). Therefore, the ventrogluteal site is the preferred injection site for infants, children, and adults (Hockenberry and Wilson, 2007; Nicoll and Hesby, 2002).
• Vastus lateralis: The vastus lateralis muscle is another injection site used in the adult client and is the preferred site for administration of biologicals (e.g., immunizations) to infants younger than 12 months of age (Nicoll and Hesby, 2002). Has small nerve endings resulting in discomfort after injection.
• Deltoid: Easily accessible muscle. Use this site only for small medication volumes (0.5 to 1 ml), and for administration of routine immunizations in toddlers, older children, and adults (Nicoll and Hesby, 2002). Give hepatitis B vaccine only in the deltoid (Beyea and Nicoll, 1995).
(Elkin, Martha Keene. Nursing Interventions & Clinical Skills, 4th Edition. Mosby, 042007. 18.104.22.168.22.214.171.124.0.1).