2.8.1 General Pharmacology

Introduction to General Pharmacology

General pharmacology introduces the core principles that govern how drugs act in the body and how the body handles drugs. For the USMLE this chapter provides the conceptual framework on which all specific drug classes rest. Later chapters on autonomic, cardiovascular, antimicrobial, and endocrine drugs will assume you know these basic ideas.

In this chapter you will meet the language of pharmacology, the logic behind dose–response relationships, and the essential rules behind drug movement, metabolism, and toxicity. The aim is not to memorize lists of drugs, but to understand patterns that apply to almost every agent you will see.

Fundamental Definitions

Before you study any specific medication you must distinguish between key terms that are often confused.

Pharmacokinetics describes what the body does to the drug. It covers absorption, distribution, metabolism, and excretion, usually abbreviated as ADME. These processes determine the concentration of drug that actually reaches the site of action over time.

Pharmacodynamics describes what the drug does to the body. It includes mechanisms of action, receptor binding, dose–response relationships, and the link between concentration and effect.

A drug is any chemical that can affect living processes. In medical practice this term usually refers to chemicals used to prevent, diagnose, or treat disease.

The therapeutic effect is the desired beneficial action of a drug. An adverse effect is any unintended, harmful response at normal doses used for prophylaxis, diagnosis, or therapy.

A side effect is an unintended effect that occurs at normal doses and may be mild or predictable, for example drowsiness with first generation antihistamines. An adverse drug reaction often implies a more severe, harmful, or unpredictable response.

Toxicity refers to harmful effects that occur when drug concentrations are too high, usually from overdose, impaired elimination, or dangerous interactions.

Receptors and Drug Targets

Most drugs act by binding to specific biological targets. The most important targets for USMLE are receptors, enzymes, ion channels, and transporters.

A receptor is a protein that recognizes specific endogenous signaling molecules and converts binding into a functional response. Drugs that act on receptors either mimic or block natural ligands.

A ligand is any molecule that binds to a receptor. Endogenous ligands include neurotransmitters and hormones. Exogenous ligands include drugs and toxins.

Affinity describes how strongly a drug binds to its receptor. High affinity means the drug binds well at low concentrations.

Efficacy describes the maximal effect a drug can produce once it is bound. It answers the question, "How much effect can this drug generate at its best?"

Drugs can also bind to targets like enzymes or ion channels without classic receptor behavior, but the core ideas of binding, selectivity, and effect still apply.

Agonists, Antagonists, and Partial Agonists

Drug receptor interactions are commonly described in terms of agonism and antagonism. These concepts are central in pharmacodynamics and appear repeatedly in questions.

An agonist binds to a receptor and activates it to produce a biological response. A full agonist has both high affinity and high efficacy and can produce the maximal possible response in that system.

A partial agonist also binds and activates the receptor but has lower efficacy. Even when it occupies all receptors it cannot achieve the same maximal effect as a full agonist.

An antagonist binds to a receptor but does not activate it. It has affinity, but no intrinsic efficacy. It reduces or blocks the effect of an agonist or an endogenous ligand.

A competitive antagonist binds reversibly to the same site as the agonist. Its effect can be overcome by increasing agonist concentration. This is important when interpreting dose–response curves and understanding clinical reversal of some drug effects.

A noncompetitive antagonist either binds irreversibly to the agonist site or to a separate allosteric site that functionally prevents activation. Its effect cannot be fully overcome by simply adding more agonist.

A partial agonist can behave as a functional antagonist when a full agonist is present, because it competes for the same receptor but produces a smaller response.

Dose–Response Relationships

To connect drug doses with clinical effects you must understand dose–response curves. These describe how the magnitude of effect changes as the dose or concentration of a drug increases.

The graded dose–response relationship looks at the response of a single tissue, cell, or patient as the dose increases. The usual plot has dose on the x axis and effect on the y axis. Because responses often change steeply over a narrow range, dose is frequently plotted on a logarithmic scale, which produces an S shaped curve.

Two important parameters arise from graded dose–response curves.

Efficacy is the maximal effect that a drug can produce, represented by the plateau at the top of the curve, often called $E_{max}$.

Potency refers to how much drug is needed to produce a specific effect. It is quantified by the concentration or dose that produces 50 percent of the maximal response. This is the $EC_{50}$ for concentration or $ED_{50}$ for dose.

In a log dose–response plot, a more potent drug lies to the left because a lower concentration achieves the same effect. A more efficacious drug has a higher plateau.

Therapeutic effects depend more on efficacy, while potency matters for how much drug must be given to achieve that effect.

Quantal dose–response relationships study all or none responses in a population of subjects. For example, at each dose you may record the percentage of patients who achieve pain relief or the percentage who develop a seizure. Quantal curves are used to define population based parameters like median effective dose and median lethal dose.

Median effective dose, $ED_{50}$, is the dose that produces a desired effect in 50 percent of the population. Median toxic dose, $TD_{50}$, is the dose that produces a toxic effect in 50 percent. Median lethal dose, $LD_{50}$, is the dose that kills 50 percent of experimental animals and is used in preclinical safety assessment.

Therapeutic Index and Drug Safety

To understand how safe a drug is you must relate effective doses to harmful doses.

The therapeutic index is a measure of drug safety obtained from quantal dose–response data.

A drug with a large therapeutic index has toxic doses much higher than effective doses. Many over the counter drugs have wide therapeutic indices. A drug with a small therapeutic index has toxic doses close to therapeutic doses and requires careful monitoring.

Clinical practice often uses the therapeutic window instead of formal $TI$. The therapeutic window is the range of plasma concentrations that yield efficacy without unacceptable toxicity. For narrow therapeutic window drugs, small changes in dose, absorption, or metabolism can cause toxicity or loss of effect. These drugs frequently require serum level monitoring.

Pharmacokinetics: ADME Overview

Pharmacokinetics describes how drug concentrations change in the body as a function of time. It answers four key questions:

How does the drug enter the body? This is absorption.

Where does the drug go in the body? This is distribution.

How is the drug chemically modified? This is metabolism, often called biotransformation.

How is the drug removed from the body? This is excretion.

Later in this chapter, each part is introduced in terms of concepts that are used across all drug classes.

Absorption and Bioavailability

Absorption is the process by which a drug leaves its site of administration and enters the systemic circulation. The route of administration heavily affects absorption characteristics.

Oral administration is convenient but subject to variable absorption and first pass metabolism in the liver before the drug reaches systemic circulation. Intravenous administration places the drug directly into the blood and bypasses absorption and first pass effect. Intramuscular and subcutaneous routes deposit drugs into tissues from which they diffuse into the bloodstream.

Bioavailability is the fraction of an administered dose that reaches the systemic circulation in unchanged form.

For orally administered drugs, bioavailability can be reduced by incomplete absorption, metabolism in the intestinal wall, or hepatic first pass metabolism before reaching the systemic circulation.

The first pass effect refers to the loss of drug as it passes through the liver after absorption from the gastrointestinal tract. Drugs with high first pass metabolism have low oral bioavailability and may require alternative routes or higher oral doses.

Several factors influence absorption, including drug solubility, formulation, gastrointestinal motility, and the presence of food. Weak acids and bases may be more or less absorbed in different pH environments due to ionization.

Membrane Transport and Ionization

To be absorbed, distributed, and eliminated, drugs must often cross biological membranes. Two fundamental properties affect this process, lipid solubility and ionization state.

Simple diffusion is the most common method of membrane crossing. Lipid soluble, nonionized drugs diffuse easily through cell membranes down their concentration gradient.

Facilitated diffusion and active transport involve carrier proteins. Facilitated diffusion moves molecules down a gradient without energy. Active transport moves molecules against a gradient using energy and often displays saturation and competition.

Weak acids and bases can exist in ionized and nonionized forms depending on pH. Only the nonionized form readily crosses lipid membranes. This leads to the concept of pH dependent ion trapping in different body compartments.

The Henderson–Hasselbalch relationship describes the ratio of ionized to nonionized forms for weak acids and bases.

For a weak acid:

$$pH = pK_a + \log\left(\frac{A^-}{HA}\right)$$

For a weak base:

$$pH = pK_a + \log\left(\frac{B}{BH^+}\right)$$

Here $HA$ is the nonionized acid, $A^-$ is the ionized conjugate base, $B$ is the nonionized base, and $BH^+$ is the ionized conjugate acid.

In acidic environments, weak acids are more nonionized and better absorbed. In basic environments, weak bases are more nonionized and better absorbed.

Clinically, manipulation of urine pH can enhance elimination of certain drugs through ion trapping. Alkalinization of urine favors excretion of weak acids by keeping them in the ionized form in tubular fluid. Acidification of urine favors excretion of weak bases.

Distribution and Volume of Distribution

After absorption, drugs distribute into body fluids and tissues. Distribution is influenced by blood flow, tissue permeability, binding to plasma proteins and tissue components, and drug lipophilicity.

Plasma protein binding, especially to albumin, can keep a portion of a drug in the vascular compartment. Only the free, unbound fraction is pharmacologically active, can cross membranes, can be metabolized, or can be excreted. For highly protein bound drugs, changes in protein levels or displacement by other drugs can significantly alter free concentrations.

The apparent volume of distribution, $V_d$, is a theoretical volume that relates the amount of drug in the body to the concentration in the plasma.

$V_d$ does not necessarily correspond to a real physical volume. Instead, it reflects how extensively a drug distributes out of the plasma into tissues.

Common interpretive patterns for $V_d$ include:

Very low $V_d$ (around 3 to 4 liters) suggests the drug is largely confined to the plasma, often because it is very large or strongly protein bound.

Intermediate $V_d$ (around 10 to 20 liters) suggests distribution into extracellular fluid.

High $V_d$ (much greater than total body water) suggests extensive distribution into tissues or fat.

$V_d$ is used in clinical calculations, such as loading dose, which is the initial dose needed to rapidly achieve a target plasma concentration.

Metabolism: Phase I and Phase II Reactions

Drug metabolism, or biotransformation, usually occurs in the liver and converts drugs into more water soluble compounds that can be excreted. Metabolism can inactivate drugs, activate prodrugs, or produce toxic metabolites.

Phase I reactions often introduce or unmask a polar functional group. Common Phase I processes are oxidation, reduction, and hydrolysis. The cytochrome P450 system is central in many oxidative reactions.

Phase II reactions conjugate the drug or its Phase I metabolite with an endogenous substrate such as glucuronic acid, sulfate, acetate, or amino acids. Conjugation reactions generally increase water solubility and promote renal or biliary excretion.

Some drugs undergo only Phase I or only Phase II metabolism, while others pass through both phases.

Metabolic capacity can vary widely among individuals because of genetic polymorphisms, liver disease, age, and interactions with other drugs. These differences can change drug clearance, effectiveness, and toxicity.

Cytochrome P450 and Drug Interactions

The cytochrome P450 enzyme family, often abbreviated CYP, performs many oxidative Phase I reactions in the liver. Different CYP isoenzymes metabolize different sets of drugs. A single drug can be a substrate, an inducer, or an inhibitor of specific CYP enzymes.

A substrate is a drug that is metabolized by a particular CYP isoenzyme.

An inducer increases the expression or activity of a CYP, which accelerates metabolism of substrates. This can lower substrate drug concentrations and reduce therapeutic effect.

An inhibitor decreases CYP activity. This slows metabolism of substrates, which can raise their concentrations and increase the risk of toxicity.

Drug interactions involving CYP enzymes are common USMLE topics. Recognizing that one drug induces or inhibits metabolism of another helps explain unexpected treatment failure or toxicity.

Excretion and Clearance

Elimination of drugs from the body occurs through metabolism and excretion. Excretion is the physical removal of unchanged drug and metabolites, mainly by kidneys in urine and by the liver in bile. Other routes include lungs, sweat, saliva, and breast milk.

Renal excretion involves glomerular filtration, tubular secretion, and tubular reabsorption. Only free, unbound drug is filtered at the glomerulus. Active secretion in the proximal tubule can transport some drugs and metabolites from blood into tubular fluid. Reabsorption in the tubules can return lipid soluble drugs to the blood, especially if they are nonionized at urinary pH.

Clearance is a key pharmacokinetic parameter that describes the volume of plasma from which a drug is completely removed per unit time.

Renal and hepatic clearances are particularly important. Impairment of kidney or liver function often decreases clearance and prolongs drug action.

In many clinical situations, knowing clearance allows you to calculate the maintenance dose needed to achieve a desired steady state concentration.

Elimination Kinetics and Half Life

Elimination kinetics describe how the rate of drug removal depends on drug concentration.

Most drugs follow first order kinetics at therapeutic doses. In first order kinetics the rate of elimination is proportional to the drug concentration. A constant fraction of drug is eliminated per unit time. On a semi logarithmic plot, concentration over time appears as a straight line.

Zero order kinetics occur when elimination mechanisms are saturated. In this case the rate of elimination is constant and independent of concentration. A constant amount of drug is removed per unit time. Zero order elimination is especially important for drugs that can easily accumulate to toxic levels when elimination pathways are saturated.

The elimination half life, $t_{1/2}$, is the time required for the plasma concentration of a drug to decrease by 50 percent. For drugs with first order kinetics, half life is constant over the therapeutic range.

There is a central relationship between half life, volume of distribution, and clearance.

Half life determines both how quickly drug levels fall after dosing stops, and how long it takes to reach steady state during continuous dosing.

For drugs given at a constant dose and interval, or by constant infusion, it takes about 4 to 5 half lives to reach steady state concentration. This rule is independent of dose. Higher doses change the steady state level, but not the time to achieve it.

Similarly, after stopping a drug, about 4 to 5 half lives are required for more than 90 percent of the drug to be eliminated from the body.

Dosing Regimens: Loading and Maintenance

Clinicians design dosing regimens using pharmacokinetic principles to achieve and maintain desired plasma concentrations without toxicity.

A loading dose is an initial higher dose that rapidly achieves a target concentration. This is useful for drugs with long half lives where waiting many days for steady state would be unacceptable. The loading dose is determined by the volume of distribution and bioavailability, as introduced earlier.

A maintenance dose is the repeated dosing required to replace the amount of drug eliminated over time and maintain a steady state concentration. Maintenance dosing is determined primarily by clearance and target concentration. In chronic dosing, steady state concentration is reached after about 4 to 5 half lives regardless of the exact dose, as long as elimination remains first order.

Dosing interval also matters. Longer intervals with larger doses can produce greater peak trough fluctuations, while more frequent dosing with smaller amounts smooths these fluctuations. For drugs with narrow therapeutic windows, minimizing swings may be important.

When organ function is impaired, dosing regimens must be adjusted. Decreased clearance typically requires lower maintenance doses or longer dosing intervals, or both. For some drugs, loading doses may remain unchanged because volume of distribution, not clearance, primarily determines loading.

Drug Response Variability

Not all patients respond to drugs in the same way. Variability in response arises from pharmacokinetic differences, pharmacodynamic differences, and patient characteristics.

Pharmacokinetic variability includes differences in absorption, distribution, metabolism, and excretion. Age, body size, organ function, concurrent drugs, and disease states can alter these processes.

Pharmacodynamic variability includes differences at the receptor or tissue level. Genetic polymorphisms can change receptor structure or number. Tolerance can develop with repeated exposure, leading to reduced effect at the same dose.

Other patient factors such as genetics, age, sex, body composition, comorbidities, diet, and environmental exposures also influence drug response. For example, liver enzyme polymorphisms can lead to fast or slow metabolism, which respectively reduce effect or increase toxicity risk at standard doses.

Recognizing that unusual responses may be due to variability rather than misdiagnosis is a key clinical skill.

Adverse Drug Reactions and Toxicity

Adverse drug reactions are unintended, harmful effects that occur at normal doses. Pharmacology questions often ask you to recognize types of adverse reactions and to distinguish predictable effects from idiosyncratic or immunologic responses.

Type A reactions are predictable and dose dependent. They are related to the known pharmacologic actions of the drug. Overanticoagulation with an anticoagulant and hypoglycemia with insulin are examples of Type A reactions.

Type B reactions are unpredictable, not dose dependent, and not directly related to the pharmacologic action. They include idiosyncratic and allergic reactions. These often occur only in susceptible individuals.

Toxicity usually refers to adverse effects from high drug concentrations, such as from overdose, drug accumulation, or interactions that reduce clearance.

For many drugs, the same receptors that mediate therapeutic effects also mediate toxicity. At higher concentrations, target tissues may be excessively affected or additional tissues may be recruited. Some drugs, however, produce toxicity through distinct mechanisms or reactive metabolites.

Drug Interactions

Drug interactions occur when the effect of one drug is altered by another substance. This can be another drug, a dietary component, or a supplement. Interactions can be pharmacokinetic or pharmacodynamic.

Pharmacokinetic interactions alter absorption, distribution, metabolism, or excretion. Examples include changes in gastric pH affecting absorption, competition for plasma protein binding, induction or inhibition of metabolic enzymes, and alteration of renal elimination.

Pharmacodynamic interactions occur when two drugs have additive, synergistic, or antagonistic effects at the same or related targets. Additive effects mean the total effect equals the sum of individual effects. Synergistic effects exceed the sum. Antagonism occurs when one drug reduces the effect of another.

Understanding these categories helps you predict whether combining drugs will enhance therapeutic benefit, increase toxicity risk, or reduce efficacy.

Special Populations and Individualization

Certain patient groups require particular care in pharmacology because their pharmacokinetics and pharmacodynamics differ from the general adult population.

In neonates and infants, organ systems are immature. Hepatic metabolism and renal excretion are reduced, and body water and fat distribution differ from adults. Doses often need to be lower on a weight basis, and some drugs are avoided entirely.

In older adults, reduced renal function is common even when serum creatinine appears normal due to lower muscle mass. Hepatic blood flow and metabolic capacity may also decline. Polypharmacy is frequent, increasing interaction risk.

Pregnant patients present concerns for both maternal pharmacokinetics and fetal exposure. Changes in plasma volume, protein binding, and renal clearance occur. Many drugs can cross the placenta, and some are teratogenic. Drug selection and dosing must consider fetal risk and altered maternal handling.

Patients with liver disease have impaired drug metabolism, especially for drugs with high hepatic extraction. Dose reduction and extended dosing intervals may be required. Patients with kidney disease have impaired excretion of many drugs and metabolites, requiring careful adjustment based on estimated renal function.

Individualization of therapy integrates these population characteristics with patient specific factors, therapeutic drug monitoring when available, and clinical response.

Tolerance, Dependence, and Addiction

Repeated exposure to some drugs leads to changes in response over time.

Tolerance is a decreased response to a drug after repeated use, requiring higher doses to achieve the same effect. Tolerance can result from pharmacokinetic changes, such as enzyme induction increasing drug clearance, or pharmacodynamic changes, such as receptor downregulation.

Dependence refers to a state in which the body adapts to the presence of a drug, such that abrupt cessation leads to withdrawal symptoms. Dependence can be physical, with clear physiological withdrawal syndromes, or psychological, with craving and compulsive drug seeking.

Addiction is a chronic, relapsing disorder characterized by compulsive drug use despite harmful consequences. It involves changes in reward pathways and behavior, and is distinct from simple physical dependence.

These concepts are essential for understanding long term therapy with drugs that act on the central nervous system, and for interpreting clinical scenarios where patients seek more medication than expected.

Conclusion

General pharmacology provides the conceptual tools to understand how drugs behave in the body and how they produce effects. The ideas of receptors, dose–response, therapeutic index, bioavailability, volume of distribution, clearance, half life, and dosing strategies appear again and again in clinical pharmacology.

As you study specific drug classes, return to these principles. They will help you make sense of why certain drugs are dosed the way they are, why some patients respond differently, and how to anticipate and manage adverse effects and interactions.