Choice of routes

The reason for choice of routes of drug administration are governing 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.

Convenienceedit

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 effectedit

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.

Oraledit

The oral route is generally the most convenient and costs the least. However, some drugs can cause gastrointestinal tract irritation. For drugs that come in delayed release or time-release formulations, breaking the tablets or capsules can lead to more rapid delivery of the drug than intended. The oral route is limited to formulations containing small molecules only while biopharmaceuticals (usually proteins) would be digested in the stomach and thereby become ineffective. Biopharmaceuticals have to be given by injection or infusion. However, recent research (2018) found an organic ionic liquid suitable for oral insulin delivery (a biopharmaceutical) into the blood stream.

Oral administration is often denoted "PO" from "per os", the Latin for "by mouth".

The bioavailability of oral administration is affected by the amount of drug that is absorbed across the intestinal epithelium and first-pass metabolism.

Localedit

By delivering drugs almost directly to the site of action, the risk of systemic side effects is reduced.

Skin absorption (dermal absorption), for example, is to directly deliver drug to the skin and, hopefully, to the systemic circulation. However, skin irritation may result, and for some forms such as creams or lotions, the dosage is difficult to control. Upon contact with the skin, the drug penetrates into the dead stratum corneum and can afterwards reach the viable epidermis, the dermis, and the blood vessels.

Mouth inhalationedit

Inhaled medications can be absorbed quickly and act both locally and systemically. Proper technique with inhaler devices is necessary to achieve the correct dose. Some medications can have an unpleasant taste or irritate the mouth.

In general, only 20–50% of the pulmonary-delivered dose rendered in powdery particles will be deposited in the lung upon mouth inhalation. The remainder of 50-70% undeposited aerosolized particles are cleared out of lung as soon as exhalation.

An inhaled powdery particle that is >8 μm is structurally predisposed to depositing in the central and conducting airways (conducting zone) by inertial impaction.

An inhaled powdery particle that is between 3 and 8 μm in diameter tend to largely deposit in the transitional zones of the lung by sedimentation.

An inhaled powdery particle that is <3 μm in diameter is structurally predisposed to depositing primarily in the respiratory regions of the peripheral lung via diffusion.

Particles that deposit in the upper and central airways are rarely absorbed systemically because they are going to be removed by mucociliary clearance in an efficient and rapid fashion.

Nasal inhalationedit

Inhalation by smoking a substance is likely the most rapid way to deliver drugs to the brain, as the substance travels directly to the brain without being diluted in the systemic circulation. The severity of dependence on psychoactive drugs tends to increase with more rapid drug delivery.

Parenteraledit

The term injection encompasses intravenous (IV), intramuscular (IM), subcutaneous (SC) and intradermal (ID) administration.

Parenteral administration generally acts more rapidly than topical or enteral administration, with onset of action often occurring in 15–30 seconds for IV, 10–20 minutes for IM and 15–30 minutes for SC. They also have essentially 100% bioavailability and can be used for drugs that are poorly absorbed or ineffective when they are given orally. Some medications, such as certain antipsychotics, can be administered as long-acting intramuscular injections. Ongoing IV infusions can be used to deliver continuous medication or fluids.

Disadvantages of injections include potential pain or discomfort for the patient and the requirement of trained staff using aseptic techniques for administration. However, in some cases, patients are taught to self-inject, such as SC injection of insulin in patients with insulin-dependent diabetes mellitus. As the drug is delivered to the site of action extremely rapidly with IV injection, there is a risk of overdose if the dose has been calculated incorrectly, and there is an increased risk of side effects if the drug is administered too rapidly.

Intranasaledit

Drug administration via the nasal cavity yields rapid drug absorption and therapeutic effects. This is because drug absorption through the nasal passages doesn't go through the gut before entering capillaries situated at tissue cells and then systemic circulation and such absorption route allows transport of drugs into the central nervous system via the pathways of olfactory and trigeminal nerve.

Intranasal absorption features low lipophilicity, enzymatic degradation within the nasal cavity, large molecular size, and rapid mucociliary clearance from the nasal passages, which explains the low risk of systemic exposure of the administered drug absorbed via intranasal.

Sublingualedit

Sublingual administration is fulfilled by placing the drug between the tongue and the lower surface of the mouth. The sublingual mucosa is highly permeable and thereby provides access to the underlying expansive network composed of capillaries, leading to rapid drug absorption.

Buccaledit

Buccally administered a medication is achieved by placing the drug between gums and the inner lining of the cheek. In comparison with sublingual tissue, buccal tissue is less permeable resulting in slower absorption.

Sublabial administrationedit

Sublabial administration