Route of administration

In pharmacology and toxicology, a route of administration is the way by which a drug, fluid, poison, or other substance is taken into the body.
Routes of administration are generally classified by the location at which the substance is applied. Common examples include oral and intravenous administration. Routes can also be classified based on where the target of action is. Action may be topical, enteral, or parenteral. Route of administration and dosage form are aspects of drug delivery.

Classification

Routes of administration are usually classified by application location.
The route or course the active substance takes from application location to the location where it has its target effect is usually rather a matter of pharmacokinetics. Exceptions include the transdermal or transmucosal routes, which are still commonly referred to as routes of administration.
The location of the target effect of active substances is usually rather a matter of pharmacodynamics. An exception is topical administration, which generally means that both the application location and the effect thereof is local.
Topical administration is sometimes defined as both a local application location and local pharmacodynamic effect, and sometimes merely as a local application location regardless of location of the effects.

By application location

Enteral/gastrointestinal route

Through the gastrointestinal tract is sometimes termed enteral or enteric administration. Enteral/enteric administration usually includes oral and rectal administration, in the sense that these are taken up by the intestines. However, uptake of drugs administered orally may also occur already in the stomach, and as such gastrointestinal may be a more fitting term for this route of administration. Furthermore, some application locations often classified as enteral, such as sublingual and sublabial or buccal, are taken up in the proximal part of the gastrointestinal tract without reaching the intestines. Strictly enteral administration can be used for systemic administration, as well as local, such as in a contrast enema, whereby contrast media are infused into the intestines for imaging. However, for the purposes of classification based on location of effects, the term enteral is reserved for substances with systemic effects.
Many drugs as tablets, capsules, or drops are taken orally. Administration methods directly into the stomach include those by gastric feeding tube or gastrostomy. Substances may also be placed into the small intestines, as with a duodenal feeding tube and enteral nutrition. Enteric coated tablets are designed to dissolve in the intestine, not the stomach, because the drug present in the tablet causes irritation in the stomach.
The rectal route is an effective route of administration for many medications, especially those used at the end of life. The walls of the rectum absorb many medications quickly and effectively. Medications delivered to the distal one-third of the rectum at least partially avoid the "first pass effect" through the liver, which allows for greater bio-availability of many medications than that of the oral route. Rectal mucosa is highly vascularized tissue that allows for rapid and effective absorption of medications. A suppository is a solid dosage form that fits for rectal administration. In hospice care, a specialized rectal catheter, designed to provide comfortable and discreet administration of ongoing medications provides a practical way to deliver and retain liquid formulations in the distal rectum, giving health practitioners a way to leverage the established benefits of rectal administration. The Murphy drip is an example of rectal infusion.

Parenteral route

The parenteral route is any route that is not enteral.
Parenteral administration can be performed by injection, that is, using a needle and a syringe, or by the insertion of an indwelling catheter.
Locations of application of parenteral administration include:

Central nervous system:
Epicutaneous. It can be used both for local effect as in allergy testing and typical local anesthesia, as well as systemic effects when the active substance diffuses through skin in a transdermal route.
Sublingual and buccal medication administration is a way of giving someone medicine orally. Sublingual administration is when medication is placed under the tongue to be absorbed by the body. The word "sublingual" means "under the tongue." Buccal administration involves placement of the drug between the gums and the cheek. These medications can come in the form of tablets, films, or sprays. Many drugs are designed for sublingual administration, including cardiovascular drugs, steroids, barbiturates, opioid analgesics with poor gastrointestinal bioavailability, enzymes and, increasingly, vitamins and minerals.
Extra-amniotic administration, between the endometrium and fetal membranes.
Nasal administration can be used for topically acting substances, as well as for insufflation of e.g. decongestant nasal sprays to be taken up along the respiratory tract. Such substances are also called inhalational, e.g. inhalational anesthetics.
Intra-arterial, e.g. vasodilator drugs in the treatment of vasospasm and thrombolytic drugs for treatment of embolism.
Intra-articular, into a joint space. It is generally performed by joint injection. It is mainly used for symptomatic relief in osteoarthritis.
Intracardiac, e.g. adrenaline during cardiopulmonary resuscitation.
Intracavernous injection, an injection into the base of the penis.
Intradermal, is used for skin testing some allergens, and also for mantoux test for tuberculosis.
Intralesional, is used for local skin lesions, e.g. acne medication.
Intramuscular, e.g. many vaccines, antibiotics, and long-term psychoactive agents. Recreationally the colloquial term 'muscling' is used.
Intraocular, into the eye, e.g., some medications for glaucoma or eye neoplasms.
Intraosseous infusion is, in effect, an indirect intravenous access because the bone marrow drains directly into the venous system. This route is occasionally used for drugs and fluids in emergency medicine and pediatrics when intravenous access is difficult.
Intraperitoneal, e.g. peritoneal dialysis.
Intrathecal is most commonly used for spinal anesthesia and chemotherapy.
Intrauterine.
Intravaginal administration, in the vagina.
Intravenous, e.g. many drugs, total parenteral nutrition.
Intravesical infusion is into the urinary bladder.
Intravitreal, through the eye.
Subcutaneous. This generally takes the form of subcutaneous injection, e.g. with insulin. Skin popping is a slang term that includes subcutaneous injection, and is usually used in association with recreational drugs. In addition to injection, it is also possible to slowly infuse fluids subcutaneously in the form of hypodermoclysis.
Transdermal, e.g. transdermal patches such as fentanyl in pain therapy, nicotine patches for treatment of addiction and nitroglycerine for treatment of angina pectoris.
Perivascular administration.
Transmucosal, e.g. insufflation of cocaine, sublingual, i.e. under the tongue, sublabial, i.e. between the lips and gingiva, and oral spray or vaginal suppository for nitroglycerine.
Topical route

The definition of the topical route of administration sometimes states that both the application location and the pharmacodynamic effect thereof is local.
In other cases, topical is defined as applied to a localized area of the body or to the surface of a body part regardless of the location of the effect. By this definition, topical administration also includes transdermal application, where the substance is administered onto the skin but is absorbed into the body to attain systemic distribution.
If defined strictly as having local effect, the topical route of administration can also include enteral administration of medications that are poorly absorbable by the gastrointestinal tract. One such medication is the antibiotic vancomycin, which cannot be absorbed in the gastrointestinal tract and is used orally only as a treatment for Clostridioides difficile colitis.

Choice of routes

The choice of routes of drug administration is governed by various factors:

Physical and chemical properties of the drug. The physical properties are solid, liquid and gas. The chemical properties are solubility, stability, pH, irritancy etc.
Site of desired action: the action may be localised and approachable or generalised and not approachable.
Rate of extent of absorption of the drug from different routes.
Effect of digestive juices and the first pass metabolism of drugs.
Condition of the patient.

In acute situations, in emergency medicine and intensive care medicine, drugs are most often given intravenously. This is the most reliable route, as in acutely ill patients the absorption of substances from the tissues and from the digestive tract can often be unpredictable due to altered blood flow or bowel motility.

Convenience

Enteral routes are generally the most convenient for the patient, as no punctures or sterile procedures are necessary. Enteral medications are therefore often preferred in the treatment of chronic disease. However, some drugs can not be used enterally because their absorption in the digestive tract is low or unpredictable. Transdermal administration is a comfortable alternative; there are, however, only a few drug preparations that are suitable for transdermal administration.

Desired target effect

Identical drugs can produce different results depending on the route of administration. For example, some drugs are not significantly absorbed into the bloodstream from the gastrointestinal tract and their action after enteral administration is therefore different from that after parenteral administration. This can be illustrated by the action of naloxone, an antagonist of opiates such as morphine. Naloxone counteracts opiate action in the central nervous system when given intravenously and is therefore used in the treatment of opiate overdose. The same drug, when swallowed, acts exclusively on the bowels; it is here used to treat constipation under opiate pain therapy and does not affect the pain-reducing effect of the opiate.