Medication and other drugs are needed by people for various reasons. A medication administration route is classified according to the location of the drug administration. They may be taken orally or by parenteral routes, which include:

• Intravenous: provides immediate onset of action.
• Intramuscular or subcutaneous: provides a slow or delayed response.

The choice of routes for drug administration depends on several factors, such as convenience, compliance, pharmacokinetics, and pharmacodynamics of the drug. In this post, you will learn more about intramuscular and subcutaneous injections.

What are intramuscular injections?

Intramuscular injection, or IM, is a technique used to administer a liquid into the depths of specific muscles in the body. Those specific muscles are highly vascularised and therefore, the administered liquid is readily absorbed into the systemic circulation, thereby bypassing the first pass effect (metabolization of orally administered drug in the body).

An IM route may be selected if oral drug absorption occurs in an irregular manner, the drug has high first-pass metabolism or if the patient is reluctant. Drugs given by this route may be either preventive or curative medicine. The most common drugs include; antibiotics, like penicillin or streptomycin, vaccines, immunoglobulins, hormones like testosterone, etc.

What are the advantages of the IM route?

1. Rapid absorption of drugs in the bloodstream.
2. Rapid onset of action of the drug.

What are the risks associated with the IM route?

Complications arise from poor practice, technique, and lack of skill.

1. Nerve injury.
2. Vascular injury.
3. Tissue death.
4. Hematoma.
5. Muscle fibrosis.

Sites of administration of IM injections:

• Dorsogluteal region.
• Ventrogluteal region.
• The deltoid muscle.
• The vastus lateralis muscle.

What are subcutaneous injections?

Subcutaneous injection is another technique to administer drugs in the body. The substance is injected into the layer of the skin called the cutis, just below the dermis and epidermis layers. The subcutaneous tissue has few blood vessels, which allows the administered drug to be absorbed at a slow and controlled rate.

A subcutaneous route may be selected if the molecular size of the drug is too large for it to be absorbed in the small intestine. Or if a faster absorption rate is needed than the oral route.

Drugs most commonly given through this route include insulin, heparin, and monoclonal antibodies.

What are the advantages of subcutaneous injections?

1. Sustained rate of absorption of the drug.
2. Fewer risks.
3. Absorption of the drug is slow thus, it is a preferable route if prolonged effect needs to be achieved.
4. Self-administration is possible.

What are the risks associated with subcutaneous injections?

1. Infection at the administration site.
2. Bruising and skin irritation.

Sites of administration of subcutaneous injections:

• Upper arm’s outer area.
• Abdomen, avoiding a 2-inch circle around the belly button.
• Front of the thigh.
• Upper back.
• Upper area of buttock, behind the hipbone.

What are the differences between IM and subcutaneous injections?

Intramuscular injections	VS	Subcutaneous injections
A higher volume of the drug can be administered.	Dose	A lower volume is administered by comparison.
Only trained and skilled personnel should administer the drug, like nurses or paramedical staff. Otherwise, it may lead to serious complications.	Skills required for administration	Easy administration. No special skill required. It can also be self-administered.
Site-specific complications. If the needle is incorrectly injected, it may lead to unresolvable paralysis and other serious complications.	Complications	Medication-specific complications. They are not as worrisome by comparison and can be resolved quickly.
Rapid absorption rate of the drug.	Absorption rate	Slow and sustained absorption rate of the drug.
Longer and thicker.	Needle size	Shorter and thinner.

What happens when intramuscular injections are given subcutaneously?

As seen earlier, IM and subcutaneous injections have their unique features which allow the administered drug to work at their optimum. If an IM vaccine is injected into the subcutaneous fat layer the absorption will be slow, and therefore the vaccine will take longer to reach the circulation. The mobilisation and processing of the antigen will be slow leading to a delayed immune response that may result in vaccine failure.

Antigens given subcutaneously remain there for longer periods of time, allowing enzymes to break them down. Vaccines containing adjuvants should be given by IM. If they are given by subcutaneous method it will lead to increased inflammation or granuloma formation.

To sum it up:

The selection of an administration route is based on the convenience of administration and the features of the drug.

Intramuscular injections administer drugs in the bulky muscles while subcutaneous injections are used to administer it just beneath the skin. The IM route is preferable if the drug needs to be rapidly absorbed, while drugs given through a subcutaneous pathway have slower, prolonged absorption.

During administering a drug via an IM route, if the needle is injected in a wrong location or incorrectly given subcutaneously it may lead to serious complications, which may or may not be resolved.

References:

https://pmj.bmj.com/content/early/2021/02/15/postgradmedj-2021-139870

https://www.pharmapproach.com/subcutaneous-route-of-drug-administration-advantages-and-disadvantages/

https://www.statpearls.com/ArticleLibrary/viewarticle/128509

https://www.statpearls.com/ArticleLibrary/viewarticle/23707