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Routes of Drug Administration

Q: What are the pros and cons of the oral route of administration?

The oral route of administration (per os) has several advantages, but also significant disadvantages. Pros: Convenience — physiological, simple for the patient, requires no sterility or medical personnel. Cons: Slow absorption — the effect does not occur immediately.Degradation — partial inactivation in the GI tract and liver (first-pass effect).Variable effect — depends on intestinal motility, mucosal condition, environmental pH, and chyme composition.Interactions — risk of interaction with food and other drugs.

For medical students2 min readUpdated 2026-10-10

Route of drug administration is the method of delivering a pharmacological agent into the body. The chosen route directly determines how quickly molecules reach the systemic circulation or lymphatic system, as well as the ultimate bioavailability and therapeutic potency.

AbsorptionThe process of a drug moving from the administration site into the systemic circulation or lymph

DynamicsThe absorption process begins immediately after medication application

PropertiesA balance of lipophilicity and hydrophilicity is crucial for successful membrane crossing

BioavailabilityThe extent of absorption determining the final magnitude of the therapeutic effect in the body

Absorption and its Pharmacokinetic Role

Absorption (absorptio, from the Latin absorbeo — to absorb) is a critical step ensuring the transfer of a medication from its initial contact site with tissues directly into the internal environment. This process begins immediately after the patient takes the drug.

It is the chosen delivery route that sets the fundamental pharmacokinetic parameters of therapy:

Rate of absorption — determines how quickly the expected therapeutic effect will occur.
Extent of absorption (bioavailability) — accounts for what fraction of the original substance reaches the target, thereby dictating the overall magnitude of the effect.
Duration of action — indicates how long the therapeutic influence on the body will persist.

Classification of Administration Routes

The fundamental division of all delivery methods is based on a single criterion: whether the drug passes through the gastrointestinal tract (GIT) or bypasses it.

Enteral Routes

The term (enteralis) derives from the Greek roots ento (inside) and enteron (gut). This option involves administering drugs through various segments of the digestive system. The main types include:

Oral (per os) — classic swallowing of the drug.
Sublingual (sublingualis) — placing the formulation under the tongue.
Buccal (transbuccalis) — applying or placing the drug on the oral mucosa against the cheek.
Rectal (per rectum) — administering the medication via the rectum.

Parenteral Routes

The second broad group combines methods (parenteralis) where the substance is delivered into the body bypassing the digestive tract. This includes inhalations, topical skin applications, and various injections:

Intravenous (intravenosa).
Intraarterial (intraarterialis).
Intramuscular (intramuscularis).
Subcutaneous (subcutanea).
Intraperitoneal (intraperitonealis).
Intrasternal (into the spongy bone of the sternum).
Intrathecal (subarachnoid or epidural administration).

Physicochemical Properties and Barriers

The choice of administration route is inextricably linked to the physicochemical properties of the molecule, primarily its polarity and solubility. Knowledge of these characteristics is critically important for designing the correct dosage form.

For a substance to be absorbed in the intestine, it must first dissolve in the liquid contents (the aqueous medium of chyme). To reach the epithelial surface, the molecule requires hydrophilicity. However, enterocyte membranes consist of a lipid bilayer. To cross this barrier via passive diffusion, the molecule must possess lipophilicity. An ideal drug features an optimal balance of these two properties.

Pharmacologists frequently utilize salt formation: administering weak acids and bases as salts dramatically increases their water solubility, thereby significantly improving bioavailability.

Effect of pH on Absorption

The pH of the environment where the drug ends up plays a decisive role in the success of absorption. According to the Henderson-Hasselbalch equation, the ionization state of a molecule changes depending on the surrounding acidity.

Non-ionized (i.e., lipophilic) fraction of the drug penetrates cellular barriers easily. Ionized molecules are retained at the membrane surface and cannot pass into the cell via simple diffusion.

Mnemonic

The term "enteral" is easy to remember through its Greek roots: ento (inside) + enteron (gut). "Parenteral" literally means "beside/beyond the gut".

Frequently asked questions

What are the pros and cons of the oral route of administration?

The oral route of administration (per os) has several advantages, but also significant disadvantages.

Pros:

Convenience — physiological, simple for the patient, requires no sterility or medical personnel.

Cons:

Slow absorption — the effect does not occur immediately.
Degradation — partial inactivation in the GI tract and liver (first-pass effect).
Variable effect — depends on intestinal motility, mucosal condition, environmental pH, and chyme composition.
Interactions — risk of interaction with food and other drugs.

What are the mechanisms of drug absorption across cell membranes?

Drug absorption across cell membranes occurs through several main mechanisms:

Passive diffusion (diffusio) — the primary mechanism for lipophilic substances along a concentration gradient.
Filtration (filtratio) — passage of small water-soluble molecules through aqueous pores.
Active transport — transfer of substances against a concentration gradient requiring energy and specific carrier systems.
Facilitated diffusion (diffusio facilitata) — an energy-independent process along an electrochemical gradient involving carrier proteins.
Pinocytosis — engulfment of large molecules with a molecular weight greater than 1,000.

How does environmental pH affect the absorption of weak acids in the stomach?

In the acidic environment of the stomach, weak acids exist predominantly in a non-ionized form, allowing their absorption to begin. As the environmental pH rises, their ionization degree increases. Ionized forms cross lipid membranes more poorly, which is why the absorption of weak acids slows down.

Which solutions must strictly not be administered subcutaneously or intramuscularly?

Subcutaneous administration strictly prohibits irritating substances and hypertonic solutions; their administration can lead to tissue necrosis (e.g., calcium chloride). Potent vasoconstrictors, such as norepinephrine, also carry a risk of tissue necrosis. The provided text contains no data on solutions strictly prohibited for intramuscular administration, noting instead that oil solutions and depot preparations can be given intramuscularly.

What are the main pharmacokinetic differences between sublingual and oral administration?

Sublingual administration differs from oral administration in the following features:

First-pass effect: with sublingual administration, the drug substance enters the systemic circulation directly, bypassing the liver; oral administration may involve partial inactivation during liver passage.
Speed of effect: with sublingual administration, absorption and onset of action occur rapidly; with oral administration, absorption is slower.
GI tract influence: sublingual substances are not exposed to gastric hydrochloric acid; oral drugs must resist GI destruction, and their absorption depends on pH, chyme composition, and food.
Absorption surface area: sublingual administration features a small absorbing surface area.

What determines the choice of a specific route of drug administration?

The choice is primarily dictated by the physicochemical properties of the active substance — its polarity and the balance of lipophilicity and hydrophilicity.

Why are weak acids and bases often formulated as salts?

Conversion into a salt increases the solubility of the molecule in an aqueous medium. This is a prerequisite for the drug to dissolve in the intestine and reach the epithelial surface.

Which fraction of a substance crosses cell membranes best?

Only the non-ionized, lipophilic fraction of a drug easily crosses the lipid bilayer of membranes.

Go deeper

Mechanisms of absorption: passive diffusion, active transport, and pinocytosis.
Impact of hepatic biotransformation on final bioavailability.
Pharmacokinetic parameters: clearance and volume of distribution.
Relationship of pharmacodynamic and toxic effects with routes of administration.
Characteristics of renal drug elimination.