Hypodermic needle

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A hypodermic needle is a hollow needle used in conjunction with a syringe to inject substances into the body. They may also be used to take liquid samples from the body, for example taking blood from a vein in venipuncture. A hypodermic needle is used when the substance would be neutralized by the digestive system, as is the case of insulin or antibiotics, if the substance needs to introduced slowly and carefully over a long period, or if the substance needs to be instantly delivered into the body. The substance can be injected just under the skin, into a muscle, or directly into the bloodstream.

Hypodermic needles, and the associated syringes, are normally disposable because re-using them or sharing them and their needles can be a means of transmission of many blood-borne diseases.

Normally made of stainless steel, the end of the needle is bevelled to create a pointed (sharp) tip, which allows it to penetrate the skin easily. When a hypodermic needle is inserted, the bevel should be facing upwards.

The size of a needle is commonly stated as a gauge, which is the outside diameter of the needle. Using the Stubs needle gauge, these range from about 7 (the largest) to 33 (the smallest). Twenty-one gauge needles are most commonly used for taking blood. The length of the needle is also stated on the packaging.

There are a number of systems for the gauging for needles, including the Stubs Needle Gauge, and the French Catheter Scale.