Injection: Treatment, Effect & Risks

Injection is the term used to describe the parenteral administration of drugs, that is, the administration of drugs bypassing the intestines. In this process, a syringe is used to deliver the drug into the skin, under the skin, into the muscle, into the vein, or into the artery.

What is injection?

In an injection, a syringe is used to put the medicine into the skin, under the skin, into the muscle, into the vein, or into the artery. The injection is usually given using a syringe with an associated needle. In contrast to infusion, the drug is administered quickly. Basically, two modes of action can be distinguished in injection therapy. On the one hand, the drug given can have a local effect. This is the case, for example, with local anesthetics. Here, the drug is usually injected subcutaneously, i.e. into the subcutaneous fatty tissue, or at nerve endings. In the case of intravenous and intra-arterial injections, the effect is systemic, as the drug is distributed throughout the body via the bloodstream. Compared to oral administration of drugs, injection therapy has several advantages. The onset of action is much faster than with oral medication. In addition, drugs can be injected that would be broken down in the gastrointestinal tract if administered orally (e.g., insulin). For orally administered agents, dosing is often difficult because absorption in the gastrointestinal tract varies from person to person. The first-pass effect is also circumvented by injection. The first-pass effect refers to the metabolism of a drug in the liver, which means that in the case of oral administration, the drug first passes through liver metabolism before it then reaches its site of action in lower concentrations. The psychological effect of an injection should also not be underestimated.

Function, effect, and goals

In everyday practice, three main types of injections are used: subcutaneous, intramuscular, and intravenous. In subcutaneous injection, the drug is applied to the subcutaneous tissue, or subcutis. The main injection sites are the upper arm, thigh or the region around the belly button. Since the subcutis consists mainly of fat cells, the administered drug is absorbed rather slowly by the body. Subcutaneous injection is therefore mainly chosen for drugs that are intended to act as a depot. An example of a drug injected subcutaneously is insulin, which is used to treat diabetes mellitus. Heparin preparations for the prevention of thrombosis are also injected subcutaneously. The procedure of subcutaneous injection is quite simple and has few complications. It can therefore also be performed by the patient himself without any problems after an introduction. In intramuscular injection, the drug is administered directly into the muscle. Preferred injection sites are the gluteus medius (gluteal muscle, the vastus lateralis muscle on the thigh or the deltoid muscle on the upper arm. To determine the correct injection site on the gluteus, the ventrogluteal method according to Hochstetter is used. With intramuscular injection, up to 20ml of a drug can be administered. The onset of action is faster than with subcutaneous injection because the muscle is better supplied with blood, but slower than with intravenous injection. Painkillers, contraceptives and cortisone preparations are mainly injected into the muscle. Vaccinations are also given as intramuscular injections. For the intravenous injection, the corresponding vein must be punctured or an already existing venous access must be used. The arm or neck veins are frequently used. The advantage of venous injection is the rapid onset of action. In addition, larger amounts of fluid can be injected via the vein. Other types of injections that are not used as frequently are intra-arterial injection (into the artery), injection into the joint capsule, intracardiac injection into the heart, injection into the bone marrow, or intracutaneous injection into the dermis.

Risks, side effects, and hazards

As mentioned earlier, subcutaneous injection is the lowest-risk injection procedure, along with intracutaneous injection. Although intramuscular injection is not difficult in its execution, it should be performed only by skilled personnel because it carries some risks. Painful and sometimes irreversible nerve injuries can occur. In addition, the penetration of pathogens into the syringe channel is feared.This is often followed by a painful syringe abscess. Another risk factor is the cannula breaking off in the muscle. This can happen particularly in patients who are in spasm. It is important to choose a sufficiently long cannula. If a cannula is used that is too short, accidental injection into fatty tissue can lead to fatty tissue necrosis. Accidental injection into a blood vessel can also have unpleasant consequences, as this allows the drug to enter the bloodstream directly in the full dosage. Therefore, a so-called aspiration in two planes is mandatory for intramuscular injections. For this, the syringe is pierced into the muscle and some aspiration is performed to see if blood flows into the syringe. If this is the case, the syringe is not in the muscle but in a blood vessel. If no blood is seen, the syringe is rotated 180 degrees and aspiration is performed again. If again no blood appears in the syringe, the drug can be injected. Absolute contraindication for intramuscular injections are patients with bleeding tendency. If a blood vessel in the muscle is injured during the placement of the injection, the resulting bleeding can hardly be stopped in patients with a bleeding tendency or on coagulant therapy (e.g., Marcumar). The two major complications of intravenous injection are paravenous injection, i.e., running next to the vein, and accidental intra-arterial injection. In both cases, severe necrosis (tissue damage) can occur. In extreme cases, complete death of the affected limb is the result.