What are the two types of injections called? Intramuscular and Subcutaneous

September 27, 2025 •

4 min read

Medications delivered via injection are a fundamental part of modern healthcare, from vaccines to insulin. To understand how they work, you must know the different methods of delivery. This guide answers, 'What are the two types of injections called?' and explores the most common types and their purposes.

Quick Summary

The two most frequently encountered types of injections are intramuscular (IM), which delivers medication deep into a muscle, and subcutaneous (SubQ), which administers it into the fatty tissue just beneath the skin. The specific type used depends on the medication and desired absorption rate, with other routes like intravenous and intradermal serving more specialized purposes.

Key Points

Intramuscular (IM) vs. Subcutaneous (SubQ): These are the two most common types of injections, differing by the tissue layer into which the medication is delivered.
Absorption Rate Varies: IM injections are absorbed faster due to richer blood supply in muscle tissue, while SubQ injections offer slower, more sustained absorption.
Specific Uses: IM injections are common for many vaccines and certain hormones, while SubQ is standard for insulin and some blood thinners.
Technique is Crucial: Needle angle, size, and injection site vary significantly between types, emphasizing the need for proper training and guidance.
Other Types Exist: Beyond IM and SubQ, intravenous (IV) and intradermal (ID) injections are used for specific medical needs, like rapid emergency delivery or diagnostic testing.
Rotate Injection Sites: For repeated injections, rotating sites prevents tissue damage and ensures consistent absorption of the medication.

Understanding the Main Types of Injections

When a medication cannot be taken orally, injections offer an effective method for delivering substances directly into the body. This process is known as parenteral administration, which bypasses the digestive system for more predictable absorption. While there are several routes, two are most commonly used in clinical and home care settings: intramuscular and subcutaneous. Understanding the distinction between these methods is key to appreciating how different medications are designed to work within the body.

Intramuscular (IM) Injections

An intramuscular injection involves delivering medication deep into muscle tissue. Muscles have a rich blood supply, which allows for faster absorption of the medication into the bloodstream compared to subcutaneous injections. This method is often chosen for medications that need to be absorbed rapidly or for those that would be irritating to inject into the fatty layer.

Common Uses:

Vaccines: Many inactivated vaccines, including the influenza and tetanus shots, are given intramuscularly.
Hormones: Certain hormonal treatments, such as testosterone and medroxyprogesterone, are administered this way.
Antibiotics: Some antibiotics are delivered via IM injection.

Common Injection Sites:

Deltoid muscle: Located in the upper arm, this is a common site for many vaccines.
Vastus lateralis muscle: This muscle is located in the thigh and is a frequent site for injections in infants and toddlers.
Ventrogluteal muscle: Located on the side of the hip, this is a safe and common site for adults, as it is free from major nerves and blood vessels.

Subcutaneous (SubQ or SQ) Injections

Subcutaneous injections are administered into the fatty tissue layer located just below the skin (the subcutis). Because this tissue has fewer blood vessels than muscle, the medication is absorbed more slowly and steadily over a period of time. This makes it an ideal route for medications that require a gradual, sustained release.

Common Uses:

Insulin: People with diabetes commonly use subcutaneous injections for insulin administration.
Blood Thinners: Medications like heparin are often administered subcutaneously.
Hormones: Some fertility medications are given this way.
Allergy Medications: Certain allergy treatments are delivered via this method.

Common Injection Sites:

Abdomen: The fatty tissue around the belly (excluding the area within two inches of the navel) is a common site for self-administered injections.
Outer thigh: The fatty tissue on the front or outer sides of the thighs can also be used.
Upper, outer arm: If someone else is administering the injection, the back of the upper arm is a suitable site.

Other Important Injection Types

Beyond intramuscular and subcutaneous, other injection routes are used for specific medical purposes:

Intravenous (IV) Injections: Medication is injected directly into a vein, providing the fastest route for the medication to enter the bloodstream. This is used for immediate therapeutic effects, such as in emergencies, or for delivering large volumes of fluids.
Intradermal (ID) Injections: Medication is injected into the dermis, the layer of skin just below the epidermis. This is the slowest absorption route and is most commonly used for sensitivity tests, such as tuberculosis (TB) and allergy testing.

Comparison of Major Injection Types

To better illustrate the differences, here is a comparison table of the most common injection routes:


Feature	Intramuscular (IM) Injection	Subcutaneous (SubQ) Injection	Intravenous (IV) Injection	Intradermal (ID) Injection
Injection Site	Deep into a muscle (deltoid, vastus lateralis)	Fatty tissue under the skin (abdomen, thigh)	Directly into a vein (arm, hand)	Dermis layer of the skin (forearm, upper back)
Absorption Rate	Fast	Slow, sustained	Instantaneous	Very slow
Needle Angle	90 degrees	45-90 degrees	25 degrees (into vein)	5-15 degrees
Needle Length	1-1.5 inches	1/2-5/8 inches	Longer, often with catheter	1/4-1/2 inches
Max Volume	Up to 4 mL (depends on muscle)	Up to 2 mL	Large volume infusions possible	Usually under 0.5 mL
Purpose	Vaccines, hormones, antibiotics	Insulin, blood thinners, certain vaccines	Emergencies, rehydration, blood products	TB testing, allergy testing

The Importance of Following Instructions

Regardless of the injection type, it is crucial to follow a healthcare provider's instructions carefully. This includes using the correct needle size, angle, and injection site. Failure to do so can lead to several complications, including nerve damage, infection, or improper medication absorption. For individuals who self-administer, proper training and guidance are essential to ensure the procedure is performed safely and effectively. Resources like the National Institutes of Health offer detailed guidelines and information on the proper administration of medication. Rotating injection sites, as advised by a doctor, is also important to prevent tissue damage and ensure consistent medication absorption over time.

Conclusion

Injections are a vital method of administering medication, with different types selected based on the specific drug, dosage, and desired speed of absorption. While many people only think of a 'shot' as one thing, the most common distinction is between intramuscular and subcutaneous deliveries. By understanding the differences and following proper technique, patients can receive their medication safely and effectively, contributing to their overall health and well-being.

Frequently Asked Questions

The primary difference is the location where the medication is injected. An intramuscular injection goes deep into a muscle, while a subcutaneous injection is given just under the skin into the fatty tissue.

Most vaccines are given via an intramuscular (IM) injection. The rich blood supply in the muscle tissue helps the medication get absorbed quickly and effectively. However, some vaccines, like the MMR, are administered subcutaneously.

An insulin shot is a classic example of a subcutaneous (SubQ) injection. This method is preferred for insulin because it allows for slower, more sustained absorption of the medication into the body.

Intravenous (IV) injections deliver medication directly into a vein for immediate effect. Intradermal (ID) injections are given just under the top layer of skin and are often used for sensitivity and allergy testing due to their very slow absorption rate.

Pain perception varies by individual. Generally, intramuscular injections can be more painful due to the deeper muscle penetration and larger needle, while subcutaneous injections are often less painful but may cause some stinging. Using proper technique can help minimize discomfort for both.

The correct injection site depends on the type. Intramuscular sites include the deltoid, vastus lateralis, and ventrogluteal muscles. Subcutaneous sites include the abdomen, outer thigh, and back of the upper arm. Your healthcare provider will provide specific instructions based on your medication.

To minimize pain, ensure the skin is clean and dry. For self-administered injections, gently pinching the skin for a subcutaneous shot or relaxing the muscle for an intramuscular one can help. Applying ice to the area before and after can also reduce discomfort and bruising.