Nurses and doctors can inject medicine directly into the blood, just under the skin or deep into the muscle. Medical professionals may choose intramuscular injections since some drugs work best that way because the drug will be absorbed at a different speed than the alternatives or because of how much fluid our muscles can hold. Speed Drugs that enter the body by intramuscular injections will be absorbed faster than subcutaneous injections, but slower than intravenous shots. Intramuscular injections may be the best choice, depending on how fast or how gradually the doctor wants the medicine to take effect. Volume The body can hold more fluid inside a muscle than it can retain just under the skin. If the syringe contains a lot of liquid, an intramuscular shot may be the best option. Drug Selection Doctors favor intramuscular injections for some drugs because muscle tissue will absorb them better than blood or the subcutanous area, aurorahealthcare.org states. Most vaccines are injected into the muscles, as are some antibiotics and contraceptives, and epinephrine for severe allergic reactions. Progesterone When women receive the hormone progesterone in their muscles during fertility treatments, it leads to more live births than vaginal progesterone treatment according to a study in the Journal of Assisted Reproduction and Genetics. The 2007 study did not determine the reason for the difference. Drawbacks Intramuscular injections shouldn't be used if the medicine will work as well given orally or intravenously, according to the Enotes Encyclopedia of Nursing and Allied Health. The site says nurses should also look for alternatives if muscles aren't big enough to absorb the drug properly. Source
