Route of administration

In pharmacology and toxicology, a route of administration is the path by which a drug, fluid, poison or other substance is brought into contact with the body.

Obviously, a substance must be transported from the site of entry to the part of the body where its action is desired to take place (even if this only means penetration through the stratum corneum into the skin). However, using the body's transport mechanisms for this purpose can be far from trivial. The pharmacokinetic properties of a drug (that is, those related to processes of uptake, distribution, and elimination) are critically influenced by the route of administration.

Classification
Routes of administration can broadly be divided into:
 * topical: local effect, substance is applied directly where its action is desired
 * enteral: desired effect is systemic (non-local), substance is given via the digestive tract
 * parenteral: desired effect is systemic, substance is given by other routes than the digestive tract

The U.S. Food and Drug Administration recognizes 111 distinct routes of administration. The following is a brief list of some routes of administration.

Topical

 * epicutaneous (application onto the skin), e.g. allergy testing, typical local anesthesia
 * inhalational, e.g. asthma medications
 * enema, e.g. contrast media for imaging of the bowel
 * eye drops (onto the conjunctiva), e.g. antibiotics for conjunctivitis
 * ear drops - such as antibiotics and corticosteroids for otitis externa
 * intranasal route (into the nose), e.g. decongestant nasal sprays
 * vaginal, e.g. topical estrogens, antibacterials

Enteral

 * by mouth (orally), many drugs as tablets, capsules, or drops
 * by gastric feeding tube, duodenal feeding tube, or gastrostomy, many drugs and enteral nutrition
 * rectally, various drugs in suppository or enema form

Parenteral by injection or infusion

 * intravenous (into a vein), e.g. many drugs, total parenteral nutrition
 * intraarterial (into an artery), e.g. vasodilator drugs in the treatment of vasospasm and thrombolytic drugs for treatment of embolism
 * intramuscular (into a muscle), e.g. many vaccines, antibiotics, and long-term psychoactive agents
 * intracardiac (into the heart), e.g. adrenaline during cardiopulmonary resuscitation (no longer commonly performed)
 * subcutaneous (under the skin), e.g. insulin
 * intraosseous infusion (into the bone marrow) 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 paediatrics when intravenous access is difficult.
 * intradermal, (into the skin itself) is used for skin testing some allergens, and also for tattoos
 * intrathecal (into the spinal canal) is most commonly used for spinal anesthesia and chemotherapy
 * intraperitoneal, (infusion or injection into the peritoneum) e.g. peritoneal dialysis is predominantly used in veterinary medicine and animal testing for the administration of systemic drugs and fluids due to the ease of administration compared with other parenteral methods

Other parenteral

 * transdermal (diffusion through the intact skin), e.g. transdermal opioid patches in pain therapy, nicotine patches for treatment of addiction
 * transmucosal (diffusion through a mucous membrane), e.g. insufflation (snorting) of cocaine, sublingual nitroglycerine, buccal (absorbed through cheek near gumline)
 * inhalational, e.g. inhalational anesthetics

Other

 * epidural (synonym: peridural) (injection or infusion into the epidural space), e.g. epidural anesthesia
 * intravitreal

Uses
Some routes can be used for topical as well as systemic purposes, depending on the circumstances. For example, inhalation of asthma drugs is targeted at the airways (topical effect), whereas inhalation of volatile anesthetics is targeted at the brain (systemic effect).

On the other hand, 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 (Narcan), 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.

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 few drug preparations suitable for transdermal administration.

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.