pharmacological medicine

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Injection (medicine)

An injection is an infusion method of putting liquid into the body, usually 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 methods of injection or infusion, including intradermal, subcutaneous, intramuscular, intravenous, intraosseous, and intraperitoneal. Long-acting forms of subcutaneous/intramuscular injections are available for various drugs, and are called depot injections.

Intravenous infusion

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).

Subcutaneous injection

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.

Intramuscular injection

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.

Depot injection

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-defense.
Some bees: poison, for self-defense and to defend their nests.
Coelenterates: poison, to kill prey.
Stinging nettles: poison, to try to avoid being eaten.

Injection pain

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. The most common technique to reduce the pain of an injection is simply to distract the patient.