General Pharmacology

Learning Objectives

By the end of this page, you should be able to:

• Define pharmacokinetics and pharmacodynamics and explain how they differ ("what the body does to the drug" vs. "what the drug does to the body").
• Trace a drug through the four ADME stages and identify the main factors that alter each stage.
• Explain first-order kinetics, half-life, and steady state, and calculate how long a drug takes to reach steady state.
• Distinguish potency from efficacy on a dose-response curve and interpret agonist/antagonist behavior.
• Calculate and interpret the therapeutic index and explain why a narrow therapeutic index changes clinical monitoring.
• Apply ADME and dose-response reasoning to real drugs (aspirin, insulin) and predict how disease states (renal/hepatic failure) change dosing.

Quick Answer

General pharmacology is the study of two linked questions: pharmacokinetics (PK) — what the body does to a drug as it moves through absorption, distribution, metabolism, and excretion — and pharmacodynamics (PD) — what the drug does to the body once it reaches its target. PK determines how much drug is present at the site of action and for how long; PD determines what that concentration actually does, from receptor binding to clinical effect. Together they explain why drugs are dosed the way they are, why some patients need higher or lower doses, why drug levels must be monitored for certain drugs, and why two drugs with the same target can have very different onset, duration, and side-effect profiles. Every rational prescribing decision — dose, route, frequency, and monitoring — is really a PK/PD decision in disguise.

Pharmacokinetics: What the Body Does to the Drug

Pharmacokinetics is usually taught as four processes, remembered by the acronym ADME: Absorption, Distribution, Metabolism, Excretion.

Absorption and Bioavailability

Absorption is the movement of a drug from its site of administration into the bloodstream. Oral drugs face the biggest obstacle course: stomach acid, gut wall enzymes, and — critically — the first-pass effect, where the liver metabolizes a large fraction of an orally absorbed drug before it ever reaches systemic circulation. This is why oral morphine requires a much higher dose than IV morphine for the same effect, and why some drugs (like nitroglycerin) are given sublingually or transdermally specifically to bypass the liver on the first pass.

Bioavailability (F) is the fraction of the administered dose that reaches systemic circulation unchanged. IV drugs have F = 100% by definition, since none is lost. Oral bioavailability is almost always lower and varies by drug (aspirin ~68%, propranolol ~25% due to heavy first-pass metabolism).

Distribution and Volume of Distribution

Once in the blood, a drug distributes into tissues. Volume of distribution (Vd) is a theoretical value describing how a dose relates to plasma concentration: Vd = Dose / Plasma concentration. It is not a real anatomical volume — it's a proportionality constant.

• A low Vd (close to plasma volume, ~5 L) means the drug stays mostly in the blood (e.g., warfarin, heavily protein-bound).
• A high Vd (much larger than total body water, sometimes hundreds of liters) means the drug has left the blood and accumulated in tissues like fat or muscle (e.g., digoxin, chloroquine).

Plasma protein binding matters here too: only the unbound (free) fraction of a drug is pharmacologically active and available to cross membranes. Conditions that lower albumin (liver disease, malnutrition) increase the free fraction of highly protein-bound drugs like phenytoin or warfarin, raising the risk of toxicity even at a "normal" total drug level.

Metabolism (Biotransformation)

Metabolism, mostly in the liver, converts drugs into forms that are usually easier to excrete.

• Phase I reactions (oxidation, reduction, hydrolysis) are largely carried out by the cytochrome P450 (CYP450) enzyme family. These reactions often produce an active or even toxic metabolite.
• Phase II reactions (conjugation — glucuronidation, sulfation, acetylation) attach a polar group to make the molecule water-soluble for excretion.

CYP450 enzymes are a favorite exam topic because they explain most clinically important drug-drug interactions: inducers (e.g., rifampin, carbamazepine, phenytoin) speed up metabolism of co-administered drugs, lowering their effect; inhibitors (e.g., ketoconazole, ritonavir, grapefruit juice via CYP3A4) slow metabolism, raising drug levels and toxicity risk.

Excretion, Clearance, and Half-Life

Excretion — mainly renal, with lesser roles for bile, lungs, and sweat — removes the drug or its metabolites from the body.

Clearance (CL) is the volume of plasma completely cleared of drug per unit time. Most drugs follow first-order kinetics: a constant fraction of the drug is eliminated per unit time, so the concentration curve is exponential.

Half-life (t½) is the time for plasma concentration to fall by 50%. This single number lets you predict two clinically vital things:

• Time to reach steady state with repeated dosing: ~4-5 half-lives, regardless of dose or frequency.
• Time to eliminate a drug after stopping: also ~4-5 half-lives to reach a clinically negligible level.

A small number of drugs (classic example: ethanol at higher doses, phenytoin, aspirin in overdose) follow zero-order kinetics, where a constant amount (not fraction) is eliminated per unit time because the metabolizing enzyme is saturated. Zero-order kinetics is dangerous because small dose increases can cause disproportionately large increases in plasma level.

Pharmacodynamics: What the Drug Does to the Body

Pharmacodynamics describes the relationship between drug concentration at the site of action and the biological effect produced — most often through binding to a receptor, enzyme, ion channel, or transporter.

Dose-Response Relationships

Plotting drug dose (or concentration, usually log-transformed) against effect gives an S-shaped (sigmoid) curve with two key parameters:

• Potency: how much drug is needed to produce a given effect. A more potent drug produces the same effect at a lower dose — reflected as the curve sitting further left. Measured as ED50 (dose producing 50% of maximal effect).
• Efficacy: the maximum effect a drug can produce, no matter how high the dose goes (Emax). Efficacy is usually the more clinically important property — a drug with lower potency but higher efficacy can still outperform a "stronger-sounding" but lower-ceiling drug.

Morphine and codeine both act on the same opioid receptor, but morphine is more potent (lower dose needed); however, a full agonist like morphine has higher maximal efficacy for pain relief than a partial agonist like buprenorphine, regardless of dose.

Agonists, Antagonists, and Partial Agonists

• A full agonist binds a receptor and produces the maximal possible response.
• A partial agonist binds the same receptor but cannot produce the full maximal response even at high concentration — and can actually reduce effect when given alongside a full agonist (by competing for the receptor).
• A competitive antagonist binds the receptor without activating it, blocking the agonist; increasing agonist dose can overcome it (shifts the curve right without lowering Emax).
• A non-competitive (irreversible) antagonist binds the receptor (often at a different site) in a way that cannot be reversed by more agonist, lowering Emax.

Therapeutic Index

The therapeutic index (TI) = TD50 / ED50 (toxic dose in 50% of subjects ÷ effective dose in 50% of subjects). A wide TI (e.g., penicillin) means a large safety margin between an effective and a harmful dose. A narrow TI (e.g., warfarin, digoxin, lithium, phenytoin, theophylline, aminoglycosides) means the effective and toxic doses are close together — these drugs require therapeutic drug monitoring (blood level checks) because small dosing errors, drug interactions, or organ dysfunction can quickly push a patient from under-treated to toxic.

Drug Classification and Routes of Administration

Drugs are grouped by chemical structure, mechanism, or therapeutic use (analgesics, antihistamines, anti-infectives, cardiovascular agents, CNS depressants/stimulants, diuretics, hormonal agents). The route of administration is chosen based on how fast an effect is needed, whether first-pass metabolism should be avoided, and patient factors:

Route	Onset	Bypasses first-pass?	Typical use
Intravenous (IV)	Immediate	Yes	Emergencies, precise titration
Intramuscular (IM)	Fast (minutes)	Yes	Vaccines, depot formulations
Subcutaneous (SC)	Slower, steady	Yes	Insulin, heparin
Oral (PO)	Slow (30-90 min)	No	Chronic, outpatient therapy
Sublingual	Fast	Yes	Nitroglycerin (angina)
Inhalation	Very fast (lungs)	Yes	Bronchodilators, anesthetics
Topical/transdermal	Slow, sustained	Yes (transdermal)	Local effect, sustained systemic release

Worked Examples

Aspirin (acetylsalicylic acid): Well absorbed orally (bioavailability ~68%), inhibits cyclooxygenase (COX-1 and COX-2) irreversibly, reducing prostaglandin and thromboxane A2 synthesis. Low doses (75-100 mg) selectively knock out platelet thromboxane A2 for the platelet's entire 7-10 day lifespan (since platelets cannot resynthesize COX) — this is why low-dose aspirin is an antiplatelet drug, while higher doses (600 mg+) are needed for anti-inflammatory/analgesic effect. At toxic doses, aspirin metabolism becomes saturated and shifts toward zero-order kinetics, which is why aspirin overdose is so dangerous and unpredictable.

Insulin: Given subcutaneously (destroyed by GI enzymes if taken orally, so it cannot be a pill), it binds the insulin receptor (a tyrosine kinase) to drive GLUT4 translocation and glucose uptake into muscle and fat. Pharmacokinetics here is dominated by formulation: rapid-acting analogs (lispro) act in minutes, while long-acting analogs (glargine) are engineered to precipitate at injection site pH and release slowly over ~24 hours — a pure pharmacokinetic manipulation used to achieve a pharmacodynamic goal (steady basal coverage).

Key Terms

Term	Definition
Pharmacokinetics (PK)	What the body does to a drug — absorption, distribution, metabolism, excretion
Pharmacodynamics (PD)	What a drug does to the body — mechanism of action and effect
Bioavailability (F)	Fraction of an administered dose reaching systemic circulation unchanged
Volume of distribution (Vd)	Theoretical volume relating total drug in the body to plasma concentration
Clearance (CL)	Volume of plasma cleared of drug per unit time
Half-life (t½)	Time for plasma drug concentration to fall by 50%
First-order kinetics	Constant fraction of drug eliminated per unit time (most drugs)
Zero-order kinetics	Constant amount of drug eliminated per unit time (saturated enzymes)
Potency	Dose needed to produce a given effect (ED50)
Efficacy	Maximum effect a drug can produce (Emax)
Therapeutic index	Ratio of toxic dose to effective dose (TD50/ED50); narrow TI needs monitoring
First-pass effect	Hepatic metabolism of an orally absorbed drug before reaching systemic circulation

Common Mistakes

Misconception 1: "A higher dose always means a stronger drug." Why it's wrong: this confuses potency with efficacy. A drug can require a small dose (high potency) but still have a lower ceiling effect (low efficacy) than a drug that needs a bigger dose. Correct: compare drugs by their maximal effect (efficacy) for the clinical outcome that matters, not by the milligrams needed to get there.

Misconception 2: "Half-life tells you how long a drug 'lasts' clinically." Why it's wrong: half-life describes plasma concentration decay, not duration of clinical effect, which also depends on receptor binding kinetics, active metabolites, and the therapeutic window. Correct: use half-life to predict time to steady state and time to elimination (~4-5 half-lives each), but judge clinical duration of action from the drug's actual PD profile (e.g., aspirin's antiplatelet effect outlasts its short plasma half-life because it irreversibly inhibits COX in platelets).

Misconception 3: "Doubling the maintenance dose will double the steady-state concentration and get me there twice as fast." Why it's wrong: doubling the maintenance dose does double the steady-state concentration, but it does NOT get you to steady state faster — time to steady state depends only on half-life, not on dose or dosing interval. Correct: to reach a therapeutic level faster, give a loading dose (calculated from Vd), not a larger maintenance dose.

Comparison and Connections

Concept A	Concept B	Key Difference
Pharmacokinetics	Pharmacodynamics	PK = body acts on drug (ADME); PD = drug acts on body (mechanism/effect)
Potency	Efficacy	Potency = dose needed; efficacy = maximum achievable effect
Competitive antagonist	Non-competitive antagonist	Competitive shifts curve right (overcome by more agonist); non-competitive lowers Emax (cannot be overcome)
First-order kinetics	Zero-order kinetics	First-order = constant fraction eliminated; zero-order = constant amount eliminated (enzyme saturation)
Bioavailability	Clearance	Bioavailability = how much drug gets in; clearance = how fast the body removes it
Full agonist	Partial agonist	Full agonist achieves maximal receptor response; partial agonist cannot, even at saturating doses

Practice Questions

Recall

1. What are the four processes of pharmacokinetics, in order? Answer guidance: Absorption → Distribution → Metabolism → Excretion (ADME).
2. Define therapeutic index and state whether a wide or narrow TI requires closer monitoring. Answer guidance: TI = TD50/ED50; a narrow TI (e.g., warfarin, lithium, digoxin) requires closer monitoring because toxic and effective doses are close.

Understanding

3. Explain why it takes roughly 4-5 half-lives to reach steady state regardless of the dose given. Answer guidance: steady state occurs when the rate of drug administration equals the rate of elimination; because elimination is a fixed fraction per half-life (first-order kinetics), the time to approach this equilibrium depends only on the elimination rate constant (half-life), not on dose size.
4. Why does a partial agonist reduce the effect of a full agonist when given together? Answer guidance: the partial agonist competes for the same receptor but produces a submaximal response even at full occupancy, so when it displaces the full agonist from some receptors, overall net effect falls below what the full agonist alone would produce.

Application

5. A drug has a half-life of 6 hours. Approximately how long after starting a fixed dosing schedule will the patient reach steady-state plasma concentration? Answer guidance: ~24-30 hours (4-5 half-lives x 6 hours).
6. A patient with liver cirrhosis and low albumin is started on phenytoin (highly protein-bound, narrow therapeutic index). What pharmacokinetic change should you anticipate, and why does it matter clinically? Answer guidance: low albumin increases the free (unbound, active) fraction of phenytoin even though the total measured level may look "normal," raising the risk of toxicity — free levels or clinical signs should guide dosing, not total level alone.

Analysis

7. Compare how a competitive antagonist and a non-competitive antagonist would each change a dose-response curve, and explain how you could tell them apart experimentally. Answer guidance: competitive antagonist shifts the curve to the right with unchanged Emax (overcome by raising agonist dose); non-competitive antagonist lowers Emax without a pure rightward shift and cannot be overcome by more agonist — distinguishable by adding increasing agonist doses and observing whether Emax is restored.
8. Aspirin overdose can shift from first-order to zero-order kinetics. Explain the mechanism and why this makes overdose management harder. Answer guidance: at toxic doses the hepatic enzymes metabolizing salicylate become saturated, so only a fixed amount (not fraction) is cleared per unit time; this means plasma levels can rise disproportionately with small additional doses and clearance no longer scales predictably, making it hard to predict how long toxic levels will persist or how much additional exposure will worsen toxicity.

FAQ

Q1: What's the simplest way to remember the difference between pharmacokinetics and pharmacodynamics? PK = "what the body does to the drug" (ADME). PD = "what the drug does to the body" (mechanism and effect). If the question is about absorption, metabolism, or how long a drug stays in the system, it's PK. If it's about receptors, mechanism of action, or effect size, it's PD.

Q2: Why do some drugs need therapeutic drug monitoring and others don't? Drugs with a narrow therapeutic index (warfarin, digoxin, lithium, phenytoin, aminoglycosides, theophylline) have effective and toxic doses close together, so blood levels are checked to keep patients in the safe window. Drugs with a wide margin (like most penicillins) don't need this because even several-fold dosing errors rarely cause harm.

Q3: Does a higher Vd mean a drug is more dangerous? Not directly — a high Vd tells you the drug has left the bloodstream and distributed into tissues (fat, muscle), which matters for dosing (loading doses) and for why hemodialysis often fails to remove high-Vd drugs in overdose (the drug isn't in the blood to be filtered).

Q4: Why does grapefruit juice interact with so many medications? Grapefruit juice inhibits intestinal CYP3A4, a major phase I metabolizing enzyme, which reduces first-pass metabolism of many oral drugs (like certain statins and calcium channel blockers) and raises their blood levels — sometimes into the toxic range.

Q5: If two drugs have the same efficacy, does it matter which one is more potent? Clinically, not much — potency mainly affects the milligram dose required, not how well the drug works. Efficacy (the ceiling effect) is what usually matters most for treatment choice, though potency can matter for formulation, cost, or pill burden.

Quick Revision

• PK = what the body does to the drug (ADME); PD = what the drug does to the body (mechanism/effect).
• ADME order: Absorption → Distribution → Metabolism → Excretion.
• Oral drugs undergo first-pass hepatic metabolism before reaching systemic circulation; IV bypasses this entirely (F = 100%).
• Bioavailability (F) = fraction of dose reaching systemic circulation unchanged.
• Volume of distribution (Vd) = Dose / Plasma concentration; high Vd = tissue accumulation, low Vd = stays in blood.
• Most drugs follow first-order kinetics (constant fraction eliminated); a few (ethanol, phenytoin, aspirin in overdose) follow zero-order kinetics (constant amount eliminated) once enzymes saturate.
• Time to steady state and time to elimination both ≈ 4-5 half-lives, independent of dose.
• Potency = dose needed for effect (ED50); Efficacy = maximum possible effect (Emax) — efficacy usually matters more clinically.
• Competitive antagonists shift the dose-response curve right (overcome by more agonist); non-competitive antagonists lower Emax (cannot be overcome).
• Therapeutic index = TD50/ED50; narrow TI drugs (warfarin, digoxin, lithium, phenytoin) need blood level monitoring.
• Low-dose aspirin is antiplatelet (irreversible COX inhibition in platelets lasting the platelet lifespan); high-dose aspirin is anti-inflammatory/analgesic.
• CYP450 inducers (rifampin, carbamazepine) lower drug levels; CYP450 inhibitors (ketoconazole, grapefruit juice) raise drug levels — a major source of drug interactions.

Related Topics

Prerequisites

• 1. Introduction to Pharmacology

Related Topics

• Drug Classification and Routes of Administration
• Adverse Drug Reactions and Drug Interactions
• Clinical Pharmacokinetics (dosing calculations, loading/maintenance doses)

Next Topics

• Autonomic Pharmacology (cholinergics, adrenergics)
• Chemotherapeutic Agents and Antimicrobials
• Toxicology and Poisoning Management