A subcutaneous (SC) injection delivers medication into the fatty tissue layer, known as the subcutis, which lies just beneath the skin. While the question of pain is common, SC injections are generally considered one of the least painful types. Most people describe the sensation as a minor pinch or pressure, similar to a quick mosquito bite. The exact feeling, however, can vary significantly depending on physical and chemical factors.
Defining Subcutaneous Injections
Subcutaneous literally means beneath the skin, targeting the adipose tissue layer located below the epidermis and dermis. This fatty layer contains fewer nerve endings and blood vessels compared to muscle tissue, which is the target of intramuscular injections. Injecting into this fat allows medication to be absorbed slowly and steadily over a prolonged period. This slow uptake is beneficial for drugs like insulin or growth hormones that require sustained release to maintain consistent therapeutic levels. Because the tissue is less dense than muscle, SC needles are characteristically short and thin, typically ranging from a half-inch to five-eighths of an inch in length.
Factors Influencing the Sensation of Pain
The discomfort experienced during a subcutaneous injection is influenced by several variables related to the injection materials and the specific medication being delivered. One key factor is the needle’s physical dimension, often measured by its gauge; a higher gauge number corresponds to a thinner needle diameter, which generally causes less initial pain upon skin penetration. The physical act of injecting a large volume of liquid can also create uncomfortable pressure within the tissue.
Studies show that pain significantly increases when the injected volume exceeds 1.0 milliliter (mL), though volumes up to 3 mL can sometimes be tolerated in areas with more fat, like the abdomen. For minimal discomfort, many injectable products are formulated to stay within a volume range of 0.5 to 0.8 mL. The temperature of the medication is another variable, as injecting a solution straight from the refrigerator can cause a sharp, stinging sensation due to the temperature difference within the body.
The chemical makeup of the drug formulation itself often determines the degree of burning or irritation after the needle is withdrawn. Solutions that have a pH level far from the body’s natural physiological pH of 7.4 can cause a localized reaction and pain. Similarly, a high osmolality—a measure of the concentration of solutes—above 600 milliosmoles per kilogram can irritate the tissue and increase discomfort. Certain preservatives, such as m-cresol, are also known to cause more injection site pain than other common excipients like benzyl alcohol.
Techniques for Minimizing Discomfort
Patients and administrators can employ several practical techniques to minimize pain during the injection process. Properly rotating the injection site is important, as repeatedly injecting into the same spot can lead to lipohypertrophy, a build-up of scar tissue that makes subsequent injections more painful. Recommended sites include the abdomen, the outer thigh, and the back of the upper arm, and these spots should be alternated with each dose.
Allowing refrigerated medication to naturally warm to room temperature for about 20 to 30 minutes before administration can reduce the stinging sensation caused by cold liquid entering the body. If the skin is cleaned with an alcohol swab, wait until the alcohol has completely evaporated before inserting the needle. Injecting through wet alcohol can push the substance into the skin, causing an avoidable burning sensation.
Injection Technique
The way the injection is performed also affects the level of discomfort. A quick, decisive needle insertion is generally less painful than a slow, hesitant one. Once the needle is fully inserted, the plunger should be depressed slowly and steadily to deliver the medication, avoiding a rapid push that creates unnecessary pressure in the subcutaneous space.
Pre-Injection Preparation
For an initial reduction in the needle stick sensation, some individuals may choose to apply ice or a topical numbing cream to the injection area for about ten to fifteen minutes beforehand.