What Is an Intramuscular Injection? Definition and What Research Reports

Plain definition

An intramuscular injection — usually abbreviated IM — is an injection in which a needle passes through the skin and the layer of fat beneath it and deposits a substance inside the body of a skeletal muscle. It sits between a subcutaneous injection, which stops in the fat layer, and an intravenous injection, which enters a vein directly. Muscle tissue carries a dense blood supply, so material placed there is generally taken up into the circulation over minutes to hours rather than instantly, and muscle can accommodate larger volumes than the skin layers above it. Vaccines, some hormone preparations, antibiotics and emergency drugs are commonly described in the literature as being given by this route.

The term in anatomical and clinical language

In clinical terminology, an intramuscular injection is a parenteral route of administration in which a bolus is deposited into the interstitial space of striated muscle. Reference material on the procedure describes the components that define it: site selection, needle length appropriate to the depth of overlying tissue, angle of entry, injected volume, and aspiration or non-aspiration technique (PMID 32310581). Because subcutaneous fat thickness varies considerably between individuals and between sites, the same needle can produce an intramuscular deposit in one person and a subcutaneous one in another — a point emphasised in evidence reviews of the technique (PMID 37357532).

Sites described in the literature

• Deltoid (upper arm) — commonly described for small-volume injections such as vaccines.
• Vastus lateralis (outer thigh).
• Ventrogluteal (lateral hip).
• Dorsogluteal (upper outer buttock) — the site most often criticised in the nursing literature.

Reviews of injection technique reported that the ventrogluteal site has been favoured over the dorsogluteal site in much of the modern evidence base, because of the proximity of major nerves and vessels to the dorsogluteal area and the variable fat depth there (PMID 25249123, PMID 18318317).

What the clinical literature reports about technique

Intramuscular administration has an unusually large procedural literature, most of it from nursing journals. A 2008 review of injection techniques examined site choice, skin preparation and needle selection and reported wide variation between published recommendations and observed practice (PMID 18318317). A 2014 evidence-based review revisited the same questions and reported continuing inconsistency in how the procedure was taught and performed (PMID 25249123). A 2023 review again explored the evidence on effective administration, indicating that the topic had not been settled by the earlier work (PMID 37357532).

Structured approaches have been proposed in response. A 2018 clinical practice guideline set out the "CLIMAT" pathway as a stepwise framework for intramuscular injection decisions in practice (PMID 29995514). On the training side, researchers described the development of an intramuscular injection simulation for nursing students, reflecting the emphasis placed on supervised skill acquisition for this route (PMID 33520557). These are professional practice documents, not instructions for lay administration.

Route matters: what comparative studies reported

The reason the term appears so often in pharmacology is that the route changes absorption. In a 2001 study in adults, researchers compared epinephrine absorption after intramuscular and subcutaneous injection and reported differences in how quickly and how highly plasma epinephrine concentrations rose depending on route and site, with intramuscular injection into the thigh producing the more rapid rise (PMID 11692118). The study is frequently cited as an illustration that "same drug, different route" is not a neutral change — it is a pharmacokinetic decision.

How the term is used in peptide research

In peptide and biologic literature, "intramuscular injection" most often appears in three contexts:

1. Immunisation studies. In one 2022 report, a mixture of a COVID-19 peptide vaccine and tetanus vaccine was administered by intramuscular injection to a horse, and the study reported induction of neutralising antibodies against authentic SARS-CoV-2 Delta variant virus (PMID 36276875).
2. Vector-delivered protein constructs. A 2020 study reported that intramuscular injection of a vectorized single-chain antibody fragment (scFvMC1) reduced pathological tau in two different tau transgenic mouse models, using muscle as the depot from which the construct was expressed (PMID 32762731).
3. Long-acting depot formulations. A 2024 formulation study examined intramuscular injection of palmitic acid-conjugated Exendin-4 loaded into multivesicular liposomes, and reported that the approach was investigated for long-acting release and improved in-situ stability of the peptide (PMID 38224039).

In each case the muscle is being used as a reservoir — a compartment from which a peptide, an antibody fragment or an antigen is released or expressed over time. That is the analytical sense in which the term appears in the research record.

Where the term is misused

• Treated as interchangeable with subcutaneous. The comparative epinephrine data show the routes are not equivalent for absorption (PMID 11692118), and a compound studied by one route has not been studied by the other.
• Assumed to be "stronger." Faster absorption in one drug class does not generalise; formulation, oil versus aqueous vehicle, and molecular size all matter, as the depot liposome work illustrates (PMID 38224039).
• Confused with cosmetic "site enhancement" oil injection. Clinical reports on intramuscular oil injection describe a distinct phenomenon with its own complication profile and are not descriptions of therapeutic administration (PMID 38445344).
• Used to imply regulatory approval. Describing a research peptide as "intramuscular" says nothing about approval status; many peptides in the literature are research-use-only materials studied in animals or in vitro, and the route of administration in a study does not confer human approval.

Related terms

Complications of intramuscular injection: What Studies Report

The procedural literature documents a recognised complication set. Reference material on the technique lists local pain, bleeding, abscess or infection, nerve injury, and inadvertent placement into subcutaneous fat or a vessel among the reported risks of the route (PMID 32310581). Reviews of administration evidence discuss site- and needle-related factors in relation to these outcomes (PMID 37357532).

Case and review literature also describes harm from non-clinical use of the route. A 2024 clinical review addressed intramuscular oil injection and the complications reported in that setting (PMID 38445344). A 2022 case report described poisoning following intramuscular injection of the pesticide methomyl, illustrating that the route can deliver toxic substances systemically (PMID 36510725).

Limitations of the evidence

Much of the technique literature is drawn from nursing practice reviews rather than randomised trials, and repeated calls for better evidence appear across 2008, 2014 and 2023 reviews (PMID 18318317, PMID 25249123, PMID 37357532). The peptide-related studies cited here were conducted in animals or in formulation models, so their findings describe those systems and not human outcomes. This page is for educational purposes only and is not medical advice; consult a licensed physician or qualified healthcare professional with any questions about medical procedures or products.

References

• Intramuscular Injection (StatPearls, 2026)
• Intramuscular oil injection (Ugeskrift for laeger, 2024)
• Intramuscular injection techniques (Nursing Standard, 2008)
• Epinephrine absorption in adults: intramuscular versus subcutaneous injection (Journal of Allergy and Clinical Immunology, 2001)
• Intramuscular injection: exploring the evidence on effective administration (Nursing Standard, 2023)
• Intramuscular injection of a mixture of COVID-19 peptide vaccine and tetanus vaccine in horse induced neutralizing antibodies against authentic virus of SARS-CoV-2 Delta variant (Vaccine: X, 2022)
• Intramuscular injection CLIMAT pathway: a clinical practice guideline (British Journal of Nursing, 2018)
• Intramuscular injection technique: an evidence-based approach (Nursing Standard, 2014)
• The Development of an Intramuscular Injection Simulation for Nursing Students (Cureus, 2020)
• Intramuscular injection of vectorized-scFvMC1 reduces pathological tau in two different tau transgenic models (Acta Neuropathologica Communications, 2020)
• A case of intramuscular injection of methomyl poisoning (Chinese Journal of Industrial Hygiene and Occupational Diseases, 2022)
• Intramuscular injection of palmitic acid-conjugated Exendin-4 loaded multivesicular liposomes for long-acting and improving in-situ stability (Expert Opinion on Drug Delivery, 2024)