IM
Meaning of IM: Two letters on a chart that decide how quickly a drug reaches the blood: into muscle, not into a vein or under the skin.
Definition of IM
IM is the abbreviation for intramuscular, meaning within or into a muscle. In clinical notes it nearly always marks a route of administration, in which a needle passes through the skin and the fat beneath it to deposit medicine in the muscle itself. The upper arm, the thigh and the buttock are the usual sites.
Why is a medicine given into muscle?
Muscle carries a rich blood supply. A drug left there is picked up steadily by the small vessels running through the tissue and carried into the general circulation. That suits two situations in particular. The first is a drug that stomach acid would destroy, or that the liver would break down before it could act, so swallowing it is not an option. The second is a volume of liquid too large to sit comfortably in the thin layer of fat under the skin. Many vaccines take this route as well, partly because muscle contains immune cells that help build the response the vaccine is meant to provoke. An intramuscular injection has to reach the muscle belly rather than stop in the fat above it, which is why the needle is longer than the one used for an injection under the skin.
How does IM compare with the other routes?
| Route | Where the dose goes | How fast it acts |
|---|---|---|
| Intravenous | Directly into a vein | Fastest; there is no absorption step at all |
| Intramuscular | Into the body of a muscle | Usually minutes, and fairly predictable |
| Subcutaneous | Into the fat just under the skin | Slower than IM, since fat carries less blood |
| By mouth | Swallowed, absorbed from the gut | Slowest and the most variable |
Speed is not the only difference. Some preparations are formulated to sit in muscle and release slowly over weeks, so the route can be chosen for a long, steady effect rather than a quick one.
What changes from one IM injection to the next?
The muscle chosen, the needle length and the volume given are not fixed. They follow the particular medication, the amount of fluid involved and the size of the person receiving it, since the depth of fat over a muscle varies a great deal from one body to another. Only a limited volume is placed in any one site, and larger amounts are split. Sites are picked to keep the needle away from large nerves and blood vessels.
Does IM always mean intramuscular?
In a drug order it does. Elsewhere the same two letters stand for other things, most often infectious mononucleosis or internal medicine, so the surrounding words settle which is meant.
Also known as
Used in a sentence
The vaccine was administered IM in the deltoid muscle.
Good to know
An IM dose generally reaches the bloodstream faster than a subcutaneous one and more slowly than an intravenous dose, which skips absorption altogether, a difference that affects how quickly the medicine takes effect.