A hypodermic needle (from Greek ὑπο- (under-), and δέρμα (skin)) is a hollow needle commonly used with a syringe to inject substances into the body or extract liquids from the body. They may also be used to take liquid samples from the body, for example taking blood from a vein in venipuncture. Large bore hypodermic intervention is especially useful in catastrophic blood loss or Shock.
A hypodermic needle is used for rapid delivery of liquids, or when the injected substance cannot be ingested, either because it would not be absorbed (as with insulin), or because it would harm the liver. There are many possible routes for an injection.
The modern hypodermic needle is generally said to have been invented in 1853, although Galen was familiar with the concept. Robert Boyle and Christopher Wren experimented with syringe-like devices in 1657, and 18th-century physician Dominique Anel created the modern pump syringe as a device to clean wounds using suction.
In all these cases, it was impossible to perform injections without an incision until Irish physician Francis Rynd invented the hollow needle in 1844. Wanting to inject of iron perchloride coagulant into an aneurysm, Charles Pravaz adapted Rynd's needle, rather than using the usual suction tube. Measuring 3 cm (1.18 in) long and 5 mm (0.2 in) in diameter, the syringe was entirely in silver, made by Établissements Charrière, and operated by a screw (rather than the plunger familiar today) to control the amount of substance injected.
Charles Pravaz did little development of the invention; it was L. J. Béhier, who made Pravaz's invention known across Europe. It thus initiated the science of sclerotherapy and the treatment of varicose veins and other varices.
Hypodermic needles are normally made from a stainless-steel tube drawn through progressively smaller dies to make the needle. The end is bevelled to create a sharp pointed tip. This lets the needle easily penetrate the skin.
The diameter of the needle is indicated by the needle gauge. Various needle lengths are available for any given gauge. There are a number of systems for gauging needles, including the Stubs Needle Gauge, and the French Catheter Scale. Needles in common medical use range from 7 gauge (the largest) to 33 (the smallest) on the Stubs scale. 21-gauge needles are most commonly used for drawing blood for testing purposes, and 16- or 17-gauge needles are most commonly used for blood donation, as they are wide enough to allow red blood cells to pass through the needle without rupturing (this also allows more blood to be collected in a shorter time). Larger-gauge needles (with smaller diameter) will rupture the red blood cells, and if this occurs, the blood is useless for the patient receiving it. Although reusable needles remain useful for some scientific applications, disposable needles are far more common in medicine. Disposable needles are embedded in a plastic or aluminium hub that attaches to the syringe barrel by means of a press-fit or twist-on fitting. These are sometimes referred to as "Luer Lock" connections.
Hypodermic needles are usually used by medical professionals (physicians, nurses, paramedics), but they are sometimes used by patients themselves. This is most common with type one diabetics, who may require several insulin injections a day. It also occurs with patients who have asthma or other severe allergies. Such patients may need to take desensitization injections or they may need to carry injectable medicines to use for first aid in case of a severe allergic reaction. In the latter case, such patients often carry a syringe loaded with epinephrine (e.g. EpiPen), diphenhydramine (e.g. Benadryl) or dexamethasone. Although sometimes disconcerting to spectators, rapid injection of these drugs may stop a severe allergic reaction.
Multiple sclerosis patients may also treat themselves by injection; several MS therapies, including various interferon preparations, are designed to be self-administered by subcutaneous or intramuscular injection.
Hypodermic needles are also used in recreational intravenous drug use. In order to limit the spread of blood-borne diseases such as hepatitis and HIV through shared injection equipment, many countries have needle exchange programs in most larger cities. In some countries, such programs are wholly or partially subsidized by the government. Before governments attained current levels of awareness about the spread of disease through shared needles, hypodermic syringes in many countries were available only by prescription; today, most countries allow hypodermic syringes to be dispensed without a prescription.
Disposable plastic syringes equipped with unbeveled blunt-tip needles are often used in industry to apply precise amounts of lubricants, adhesives, and various other liquids and pastes.
It is estimated that about 10% of the adult population may have a phobia of needles (trypanophobia), and it is much more common in children, ages 5-16. Children, or even adults, can ask for a patch from the nurse to numb the area of where the injection will take place to reduce pain.