An injection delivers drug directly into the body, bypassing the digestive system. That makes it fast, predictable, and useful for drugs that would be destroyed by stomach acid or poorly absorbed. There are several routes. Intravenous injection goes into a vein and acts within seconds. Intramuscular injection goes into a muscle, like the deltoid or thigh, and absorbs over minutes to hours. Subcutaneous injection goes into the fat layer under the skin and absorbs more slowly. Intradermal injection goes into the top layer of skin, used for some vaccines and allergy tests.

Each route has its own equipment and technique. IV injections use a needle or catheter placed in a vein. IM injections use a longer needle to reach muscle. Subcutaneous injections use a short, thin needle. The volume that can be injected varies: IV can handle large volumes, while subcutaneous is limited to a few milliliters. Sterile technique is essential. Needle-stick injuries and infections are preventable with proper precautions.

Common considerations:

• Route. IV, IM, subcutaneous, intradermal.
• Onset. IV is fastest, subcutaneous is slowest.
• Volume. IV can take large volumes; subcutaneous cannot.
• Site rotation. Repeated injections in the same spot can cause tissue damage.

Patients with diabetes, rheumatoid arthritis, and other chronic conditions often self-inject at home. Training and proper disposal of sharps are important.