
Originally Posted by
Othello
fuond this about needle size for IM:
Intramuscular Injections
Intramuscular injections go into the muscle below the subcutaneous layer, so the needle must be thicker and longer to ensure that the medicine is being injected into the proper tissue. Twenty (20) or 22 G needles that are an inch or an inch and a half long are usually appropriate for this type of injection.
A person who is thin, with very little fatty tissue can use the inch long needle; a heavier person may need to use the inch and a half long needle.
so 5 or 6 mm needles that come with pens cannot reach the muscle tissue ... (for reference 5mm = 0.19685 inches)