Sterile technique and injection-site infection

• A new, sterile, single-use needle every time: never reuse a needle and never share equipment. Reuse dulls the tip and reintroduces bacteria.
• Clean hands before you start; clean the vial top and the injection site with a fresh 60–70% alcohol swab and let it fully dry before the needle goes in.
• Draw with one tip and inject with a fresh one where practical, drawing through a rubber stopper blunts the needle.
• Inject slowly, and dispose of sharps in a proper container, never a household bin.

Intramuscular injections go into large muscles with room away from major nerves and vessels, commonly the ventrogluteal site, the deltoid, and the vastus lateralis. Each is found from bone, not by eye: the ventrogluteal site is on the side of the hip, in the triangle between the iliac crest and the greater trochanter, and is generally considered the safest because the muscle is thick there and the fat layer over it is thin; the deltoid band sits a hand’s width below the point of the shoulder, clear of the radial and axillary nerves; the vastus lateralis is the middle third of the line from the greater trochanter down to the outer knee. Subcutaneous injections go into the fat layer instead, the abdomen at least 2 in (5 cm) from the navel, the outer thigh, or the back of the upper arm. Whichever route, rotating sites gives each one time to heal and prevents scar tissue and abscesses from building where the same spot is hit repeatedly.

• Rotate between and within sites so no single spot is injected repeatedly. The usual guidance is to move at least 1 in (2.5 cm) from the last injection.
• Avoid injecting into scar tissue, moles, broken skin, or an area that is already red, hot, or swollen.
• Aim away from nerves, visible vessels, and bone; learn the landmarks for whichever site you use.
• Volume matters to site choice: the deltoid is a small muscle and takes correspondingly small volumes, while the vastus lateralis and gluteal sites take larger ones.

A little soreness or a small bruise at a fresh injection site is ordinary. An infection is different: it grows rather than fades, and it brings warmth, spreading redness, and swelling. Catching it early is the difference between antibiotics and a drained abscess.