The sensation of pain during an injection is highly personal, influenced by psychological factors such as anxiety and expectation. However, the intensity of a shot’s sting is also determined by several physical and chemical factors.
Categorizing Pain by Injection Type
The depth and location of the needle’s placement are primary determinants of how much an injection hurts, as different tissues possess varying densities of nociceptors. The three main types of injections target distinct layers beneath the skin, each with a unique pain profile.
An Intradermal (ID) injection is delivered just beneath the epidermis, the skin’s outermost layer. This area is rich in superficial nerve endings, and the injection often causes a localized, sharp, burning sensation and a visible welt, or wheal. Though the volume is tiny (typically 0.1 milliliters (mL)), the proximity to the skin’s sensitive surface makes it an acutely painful, momentary experience.
A Subcutaneous (SC) injection places the substance into the fatty layer directly beneath the skin. This fatty tissue has fewer nerve endings compared to the deeper muscle, generally making the initial needle stick less painful than an ID or intramuscular injection. However, if the volume of the substance exceeds about 0.5 to 0.8 mL, the pressure of the fluid expanding the tissue can become the primary source of discomfort. The maximum volume typically tolerated in the subcutaneous layer is around 1.5 mL.
Intramuscular (IM) injections deliver the substance deep into the muscle tissue, which is less sensitive than the skin’s surface but can result in significant post-injection soreness. The pain felt is less about the initial needle insertion and more about the potential irritation and inflammation caused by the fluid within the muscle fibers. Choosing the injection site can also affect pain; for instance, in children, the ventrogluteal region has been shown to be less painful than the vastus lateralis (outer thigh) area for IM shots.
The Chemical Factors That Increase Pain
Beyond the needle’s mechanics, the properties of the injected liquid often cause the most severe, burning, or lingering pain.
A major contributor to injection pain is the solution’s pH, or its acidity or alkalinity. The body’s physiological pH is tightly regulated around 7.4, and solutions formulated with a pH far outside this neutral range directly irritate tissue and activate pain receptors. Strongly acidic or alkaline drugs, such as certain antibiotics or local anesthetics without buffering agents, can disrupt the local chemical environment, leading to a noticeable burning sensation. To minimize pain, injectable products are ideally formulated to have a pH close to the body’s natural level.
Another factor is the solution’s osmolality, which is a measure of the concentration of solutes in the fluid. Solutions that are hyperosmolar—meaning they have a much higher solute concentration than the body’s fluids—can draw water out of cells, causing them to shrink and leading to irritation and tissue injury. Ideally, injectable products should be isotonic, with an osmolality of about 300 mOsm/kg, though up to 600 mOsm/kg may be tolerated without significantly increasing pain.
The volume of the fluid is also directly related to pain, especially in IM and SC injections. Larger volumes physically distend or stretch the tissue, activating pressure receptors and causing discomfort. Minimizing the volume delivered at one time reduces post-injection soreness. Furthermore, the viscosity, or thickness, of the liquid can increase the pressure required for injection, which may displace tissue and contribute to the overall feeling of discomfort.
The temperature of the solution can affect comfort, as most injectable medications are stored in a refrigerator. Injecting a cold solution can cause vasoconstriction and discomfort compared to a solution warmed to room temperature. Allowing a refrigerated medication to sit for 20 to 30 minutes to reach room temperature before administration can reduce the sting.
Techniques for Minimizing Discomfort
While the physical and chemical properties of a shot are fixed, several techniques can be employed to reduce the discomfort experienced.
Managing psychological factors, such as anxiety, is a powerful first step, as a relaxed state prevents muscles from tensing up, which can intensify pain. Controlled breathing and distraction, such as listening to music or engaging in conversation, can help manage the perception of pain by shifting focus away from the injection site.
Physical techniques can prepare the injection site to minimize the sting. Applying a topical anesthetic cream beforehand or using an ice pack to numb the skin for about 15 minutes can temporarily reduce the sensation of the needle. For ongoing self-injections, consistently rotating the injection site prevents scar tissue formation and reduces localized tenderness from repeated punctures.
Providers can also employ techniques to improve the experience. Using the smallest appropriate needle gauge and injecting the substance with a steady, rather than rapid, pace helps prevent excessive tissue pressure and irritation. A swift, decisive insertion of the needle is generally less painful, as it activates fewer nociceptors upon entry.