Basic Principles of Pharmacology

Core Concepts

Pharmacokinetics: what the body does to the drug

Absorption depends on the route. Oral drugs must survive the digestive tract and pass through the liver before reaching circulation — the first-pass effect — which can destroy much of the dose. Intravenous injection bypasses absorption; inhalation reaches the blood quickly; transdermal patches deliver slowly. Distribution carries the drug through the bloodstream, but the blood–brain barrier (BBB) blocks many molecules from the brain. Metabolism — mostly in the liver via cytochrome P450 enzymes — converts drugs into water-soluble forms for excretion by the kidneys. Half-life (time for concentration to fall by half) determines dosing frequency.

The blood–brain barrier: the brain's bouncer

The BBB is formed by tight junctions between capillary endothelial cells plus supporting cells (astrocytes, pericytes). It blocks large or water-soluble molecules, while small, lipid-soluble ones pass by diffusion — which is why most psychoactive drugs are designed to be lipophilic. The barrier also actively pumps some drugs out of the brain via transporters such as P-glycoprotein. A drug that works in a test tube may fail in the brain simply because it never arrives.

Pharmacodynamics: what the drug does to the body

Most psychoactive drugs act at receptors — neuronal proteins that normally bind endogenous signaling molecules (ligands) such as neurotransmitters. Two properties matter most. Affinity is how tightly the drug binds; efficacy is how strongly it changes receptor activity once bound. A full agonist produces a strong response, mimicking the natural ligand; a partial agonist produces a weaker response; an antagonist blocks the natural ligand; an inverse agonist does the opposite of the natural ligand. Drugs can also act indirectly — blocking reuptake transporters, inhibiting degrading enzymes, or altering release — rather than binding the receptor itself.

Dose–response relationships

As dose increases, effect grows until a plateau. The ED50 is the dose producing a specified effect in 50% of a population; the LD50 is the dose lethal to 50%. Their ratio, the therapeutic index, is the safety margin: small index = narrow window between therapeutic and toxic. Potency (dose needed for an effect) is often confused with efficacy (maximum effect achievable) — a highly potent drug is not necessarily more effective.

Tolerance, dependence, and withdrawal

With repeated use, the body adapts. Metabolic tolerance means the liver processes the drug faster; pharmacodynamic tolerance means receptors or signaling adjust (desensitization, downregulation), so the same dose has less effect. Dependence is the adapted state in which normal function requires the drug; stopping abruptly produces withdrawal. Cross-tolerance extends to other drugs in the same class. These adaptations explain why chronic users need higher doses and why withdrawal mirrors the acute effect.

How It Works / Step-by-Step Process

Journey of an oral psychoactive drug:

1. The pill is swallowed and dissolves; the drug is absorbed across the gut into the portal blood.
2. The blood passes through the liver, where first-pass metabolism destroys a portion of the dose (the rest reaches general circulation).
3. The drug circulates; being small and lipid-soluble, it crosses the BBB into the brain.
4. At the synapse, it binds its receptor — activating it (agonist), blocking it (antagonist), or acting indirectly (e.g., blocking a reuptake transporter).
5. Receptors transduce the signal; neuronal firing and downstream signaling change; behavior, mood, or cognition shifts.
6. Meanwhile the liver metabolizes the drug into water-soluble forms and the kidneys excrete them; after one half-life, half the drug is gone — the cycle repeats at the next dose.

Common Confusions