Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
An injection is a method of putting liquid into the body with a hollow needle and a syringe which is pierced through the skin to a sufficient depth for the material to be forced into the body. An injection follows a parenteral route of administration, that is, its effect is not necessarily local to the area in which the injection is administered; it is systemic.
There are several types of injections or infusions, including intradermal, subcutaneous, and intramuscular injections, and intravenous or intraosseous infusions. Long-acting forms of subcutaneous/intramuscular injections are available for various drugs; these are called depot injections.
An intravenous infusion is a liquid administered directly into the bloodstream via a vein. The first polio vaccine in 1952 was injected intravenously until an oral vaccine replaced it in 1955.
When a rapid onset of action is needed, medications may be administered intravenously, intramuscularly, under the tongue (sublingual), or by intranasal or oral inhalation (insufflation).
A subcutaneous injection is administered as a bolus into the subcutis, the layer of skin directly below the dermis and epidermis, collectively referred to as the cutis. Subcutaneous injections are highly effective in administering vaccines and such medications as insulin, morphine, diacetylmorphine or goserelin.
A person with Type I diabetes mellitus typically injects insulin subcutaneously. Places on the body where people can inject insulin most easily are:
- The outer area of the upper arm.
- Just above and below the waist, except the area right around the navel (a 2-inch circle).
- The upper area of the buttock, just behind the hip bone.
- The front of the thigh, midway to the outer side, 4 inches below the top of the thigh to 4 inches above the knee.
These areas can vary with the size of the person.
Changing the injection site keeps lumps or small dents called lipodystrophies from forming in the skin. However, people should try to use the same body area for injections that are given at the same time each day-for example, always using the abdomen for the morning injection or an arm for the evening injection.
Using the same body area for these routine injections lessens the possibility of changes in the timing and action of insulin.
In an intramuscular injection, the medication is delivered directly into a muscle. Many vaccines are administered intramuscularly, as well as codeine, metoclopramide, and many other medications. Many drugs injected intramuscularly are absorbed into the muscle fairly quickly, while others are more gradual.
Generally, intramuscular injections are not self-administered, but rather by a trained medical professional. However, prescribed self-administered intramuscular injections are becoming more common for patients that require these injections routinely.
A depot injection is an injection, usually subcutaneous or intramuscular, of a pharmacological agent which releases its active compound in a consistent way over a long period of time. Depot injections are usually either solid or oil-based. Depot injections may be available as certain forms of a drug, such as decanoate salts or esters. Examples of depot injections include Depo Provera and haloperidol decanoate.
The advantages of using a long-acting depot injection include increased medication compliance due to reduction in the frequency of dosing, as well as more consistent serum concentrations. However, one significant disadvantage of using a depot injection is that the drug is not immediately reversible, since it is slowly released.
Hypodermic injections in nature
Various animals, and some plants, have been injecting for various reasons long before humans began doing so. This process is often called stinging. Some examples include:
- Snakes, wasps, scorpions: poison, to kill prey and self-defence.
- Some bees: poison, for self-defence and to defend their nests.
- Coelenterates: poison, to kill prey.
- Stinging nettles: poison, to try to avoid being eaten.
The pain of an injection may be lessened by prior application of ice or topical anesthetic or simultaneous pinching of the skin. Recent studies suggest that forced coughing during an injection stimulates a transient rise in blood pressure which inhibits the perception of pain. Sometimes, as with an amniocentesis, a local anesthetic is given.  Perhaps the most common technique to reduce the pain of an injection is simply to distract the patient.
- ↑ Anesthesia and Analgesia 2004;98:343-5
- Intracardiac injection
- Intraosseous infusion
- Occupational Safety and Health Administration (OSHA)
- FDA Center for Drug Evaluation and Research Data Standards Manual: Route of Administration.
- Subcutaneous Injection Sites - from healthinfotranslations.com
- Giving Medicine by Subcutaneous Injection - from healthinfotranslations.com
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