Absorption

Absorption is the movement of a drug from its administration site — the gut, muscle, skin, or elsewhere — across biological membranes into the blood. Every oral, subcutaneous, intramuscular, transdermal, and inhaled drug must be absorbed; only intravenous administration skips this step entirely, depositing the drug directly into the bloodstream at 100% bioavailability.

Mechanisms of Crossing Membranes

Most drugs cross cell membranes by passive diffusion, moving from an area of high concentration to low concentration without using cellular energy. This process depends heavily on lipid solubility: cell membranes are phospholipid bilayers, so lipid-soluble (lipophilic) drugs pass through easily, while water-soluble (hydrophilic) drugs struggle unless a transporter helps them.

Ionization is the other major variable. Many drugs are weak acids or weak bases that exist in equilibrium between an ionized (charged) form and a nonionized (uncharged) form, depending on the surrounding pH. Only the nonionized form is lipid-soluble enough to diffuse across membranes efficiently; the ionized form gets trapped in whatever compartment it's in. This is why gastric pH matters: weak acids (like aspirin) are more nonionized, and thus more absorbable, in the acidic stomach, while weak bases absorb better in the more alkaline small intestine. This pH-partitioning principle also explains "ion trapping" in overdose management, though the clinical detail there is beyond this scope.

A smaller subset of drugs, along with nutrients the body needs regardless of concentration gradient (glucose, amino acids, some vitamins), use active transport via specific carrier proteins. Active transport can move substances against a concentration gradient, requires energy, and is saturable — meaning it can be overwhelmed at high concentrations, unlike passive diffusion which has no such ceiling under normal conditions.

Factors That Speed or Slow Absorption

Several physiological variables modulate how quickly and completely a drug is absorbed, independent of the mechanism used:

• Surface area: The small intestine, with its vast villi and microvilli, offers enormous surface area and is the primary absorption site for most oral drugs — far more efficient than the stomach.
• Blood flow (perfusion): Highly vascular tissues absorb drugs faster because the bloodstream continuously carries drug away, maintaining the concentration gradient that drives diffusion. This is why intramuscular injections into a well-perfused muscle absorb faster than subcutaneous injections into fattier, less vascular tissue.
• Gastric emptying: Since the small intestine is where most absorption happens, how quickly the stomach empties its contents into the duodenum directly affects how fast an oral drug reaches its main absorption site. Anything that speeds gastric emptying (an empty stomach, prokinetic drugs) tends to speed absorption; anything that slows it (high-fat meals, opioids, delayed-release formulations) tends to delay it.
• Food effects: Food can slow gastric emptying, bind or dilute a drug, alter gastric pH, or in some cases enhance absorption of lipophilic drugs by stimulating bile release. Effects are drug-specific — some medications must be taken on an empty stomach for adequate absorption, others are recommended with food to reduce GI irritation or improve uptake.

First-Pass Metabolism and Bioavailability

Bioavailability is the fraction of an administered dose that reaches systemic circulation unchanged and able to exert an effect, expressed as a percentage. IV drugs have 100% bioavailability by definition; every other route has bioavailability at or below 100%.

The major reason oral bioavailability is often reduced is first-pass metabolism (the first-pass effect). Drugs absorbed from the stomach and intestines enter the portal venous circulation, which routes blood directly through the liver before it reaches the rest of the body. The liver's metabolic enzymes can inactivate a substantial fraction of the drug before it ever reaches systemic circulation — sometimes so much that an oral formulation is impractical and the drug must be given by a route that bypasses the portal system.

Route Comparisons

Route of administration is one of the biggest determinants of both absorption rate and extent:

• Intravenous: No absorption phase; immediate, complete systemic availability. Fastest onset, but least forgiving of dosing errors.
• Intramuscular and subcutaneous: Absorption depends on blood flow to the injection site; generally slower than IV but avoids first-pass metabolism.
• Oral: Convenient but subject to variable gastric emptying, gut pH, food interactions, and first-pass metabolism, making it the least predictable route in terms of both rate and extent.
• Sublingual and buccal: Drug absorbs directly into the venous system draining the mouth, bypassing both the gut and the liver's first pass — useful for drugs that need rapid onset or are heavily degraded by first-pass metabolism.
• Transdermal and inhaled: Transdermal absorption is slow and steady, useful for sustained delivery; inhaled drugs reach a huge, highly vascular pulmonary surface area, enabling very rapid systemic or local absorption.

Understanding absorption sets up everything that follows in pharmacokinetics: a drug that isn't absorbed adequately can never be distributed, metabolized, or excreted in a way that produces a therapeutic effect.