Injection into muscle

Intramuscular (also IM or im) injection is the injection of a substance directly into muscle. In medicine, it is one of several alternative methods for the administration of medications (see route of administration). Muscles have larger and more blood vessels than subcutaneous tissue and injections here usually have faster rates of absorption than subcutaneous injections or intradermal injections. Depending on the injection site, an administration is limited to between 2 and 5 milliliters of fluid.

Examples of medications that are sometimes administered intramuscularly are:

In addition, some vaccines are administered intramuscularly:

Platelet-rich plasma injections can be administered intramuscularly.

Certain substances (e.g. ketamine) are injected intramuscularly for recreational purposes.

Possible sites for IM injection include: deltoid, dorsogluteal, rectus femoris, vastus lateralis and ventrogluteal muscles. Sites that are bruised, tender, red, swollen, inflamed or scarred are avoided.

The deltoid muscle site (upper arm) is recommended for use with injections of small volume, usually equal or less than 1 ml, including vaccinations. This site is not recommended for repeated injections, due to its small area, it is difficult to rotate the injection site. To locate the site, palpate the lower edge of the acromion process. Inject in the upside down triangle that forms with its base at the acromion process and its midpoint in line with the axilla.

...
Wikipedia