A pen needle is a sterile, disposable accessory designed to attach to a medication pen, resembling a large writing instrument. Its function is to facilitate the subcutaneous delivery of liquid medication, such as insulin or growth hormone, directly under the skin’s surface. This device makes self-injection simpler and more precise than traditional syringe-and-vial methods. Pen needles are single-use items that maintain sterility until they are attached for injection.
Anatomy and Mechanism
The pen needle uses a specialized, dual-ended cannula, the hollow tube that transports the medication. The injection needle penetrates the patient’s skin to deliver the dose. The opposite end, the cartridge needle, is shorter and pierces the rubber stopper on the medication cartridge inside the pen. This dual design creates a sealed channel for the drug to flow from the reservoir into the subcutaneous tissue.
The center of the needle assembly is the hub, a plastic piece that securely screws or pushes onto the threads of the medication pen. Before use, the assembly is protected by two caps: a removable outer shield and a smaller, inner cap covering the injection needle. These protective layers maintain sterility and prevent accidental needle-stick injuries.
Modern pen needles incorporate features for patient comfort and efficient drug delivery. Many feature thin-wall technology, which allows for a wider inner diameter despite a fine outer gauge, improving the flow rate for viscous medications. The injection needle is typically coated with a thin layer of medical-grade silicone lubricant to minimize friction. The needle tip is often designed with a multi-bevel point to create a smoother penetration site.
Needle Sizing and Selection
Pen needles are categorized by length, measured in millimeters (mm), and gauge (G), which refers to thickness. Lengths typically range from 4 mm to 12.7 mm, but current clinical practice favors shorter needles (4 mm, 5 mm, or 6 mm) for most adults and children. The goal is to reach the subcutaneous fat layer, and shorter needles are effective because human skin thickness is relatively consistent.
Using a short needle reduces the risk of inadvertently injecting medication into the muscle, which can affect drug absorption and cause pain. When using very short needles, a straight 90-degree insertion angle is usually recommended without needing to pinch the skin. Longer needles, such as 8 mm or 12.7 mm, often require a skin fold or a 45-degree angle to ensure subcutaneous delivery.
Gauge measures the outer diameter of the needle and follows an inverse scale: a higher gauge number signifies a thinner needle. Common gauges range from 29G to 34G, with 32G and 34G being ultra-fine options. Thinner needles generally correlate with less pain during injection. However, a thinner needle may slightly increase the time required to push the medication through. The appropriate length and gauge should always be determined in consultation with a healthcare provider, considering factors like injection volume and body mass index.
Safe Injection Technique and Disposal
Proper technique begins with attaching a new, sterile needle for every injection. After twisting the hub onto the pen, remove the outer and inner caps to expose the injection needle. A small test dose, called “priming,” is performed by dialing up two units of medication and expelling it into the air. This confirms the needle is open and the flow path is clear before the full dose is administered.
The injection should be performed quickly, inserting the needle at a 90-degree angle for the shortest sizes or a 45-degree angle for longer needles, if advised. Once inserted, the dose is pushed, and the pen’s button must be held down for 6 to 10 seconds before the needle is withdrawn. This crucial holding time ensures complete delivery of the medication and minimizes the possibility of the drug leaking back out of the injection site.
A pen needle must never be reused, as this dulls the tip, increases the risk of infection, and may block the fine cannula. Immediately following the single use, the needle must be safely removed from the pen and discarded. Used needles must be placed into a designated, puncture-resistant sharps container, such as an FDA-cleared device or a heavy-duty plastic container with a secure lid. Never place used needles loosely into household trash, as this poses a significant risk of accidental injury and transmission of bloodborne pathogens.