Every pill, injection, or tablet that reaches a patient’s hands is more than just a chemical compound — it’s the result of centuries of scientific exploration in the field of pharmacology. From understanding how drugs act on the body to predicting how the body processes them, pharmacology stands at the heart of modern medicine.
This unit, General Pharmacology, introduces the foundational concepts that govern drug action, pharmacokinetics, and the body’s varied responses. It also explores the nature and sources of drugs, their administration routes, and how the science of drug behavior ensures safety and efficacy in therapy.
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Introduction to Pharmacology
Definition and Scope
Pharmacology is the branch of science that studies drugs and their interactions with living systems. It examines how drugs produce their effects (pharmacodynamics) and how the body handles these substances (pharmacokinetics).
The scope of pharmacology extends from the discovery of new molecules to their testing, therapeutic application, and monitoring in clinical use. It bridges chemistry, physiology, and medicine, serving as a foundation for rational drug design and safe therapy.
Historical Landmarks
Pharmacology’s history stretches back to ancient civilizations, where plant and animal extracts were used as medicines. However, it was during the 19th and 20th centuries that pharmacology became a scientific discipline. The isolation of morphine, discovery of penicillin, and development of synthetic drugs marked the beginning of modern therapeutics.
Today, pharmacology plays a crucial role in drug innovation, personalized medicine, and understanding molecular targets within the human body.
Nature and Sources of Drugs
Drugs can originate from natural, synthetic, or biotechnological sources.
Natural sources include plants (e.g., morphine from Papaver somniferum), animals (e.g., insulin from pancreas), and minerals (e.g., iron supplements).
Synthetic drugs are chemically designed in laboratories, such as paracetamol or sulfonamides.
Biotechnological drugs, like monoclonal antibodies and vaccines, represent the frontier of modern pharmacology.
The essential drugs concept, introduced by the World Health Organization (WHO), ensures that every healthcare system maintains a list of safe, effective, and affordable drugs necessary for meeting public health needs.
Routes of Drug Administration
The route by which a drug enters the body significantly influences its absorption, onset, and duration of action.
Oral Route: Most common, convenient, and economical. However, it’s affected by food and first-pass metabolism.
Parenteral Route: Includes injections (intravenous, intramuscular, subcutaneous) for faster and controlled drug delivery.
Topical and Transdermal Routes: For local action on skin, eyes, or mucosa.
Inhalation Route: Provides rapid absorption through the lungs, often used in asthma therapy.
Selecting the appropriate route is a crucial part of clinical decision-making to achieve desired therapeutic effects.
Pharmacodynamic Concepts: How Drugs Act
Agonists and Antagonists
Drugs act by binding to specific cellular proteins known as receptors.
Agonists are drugs that mimic natural substances and activate receptors to produce a response.
Antagonists block the receptor and prevent activation by agonists.
When antagonism occurs at the same receptor site, it’s competitive; if it occurs through different mechanisms or sites, it’s non-competitive.
Spare Receptors
Sometimes, a full response can be achieved even when not all receptors are occupied. These spare receptors enhance the sensitivity of the system to a drug, allowing lower doses to achieve maximum effect.
Drug Tolerance and Dependence
Repeated exposure to certain drugs may lead to tolerance, where higher doses are required to achieve the same effect. Prolonged use can cause dependence, leading to withdrawal symptoms when the drug is stopped.
Addiction refers to compulsive drug-seeking behavior, often psychological.
Tachyphylaxis is a rapid decrease in response after repeated doses in a short period.
Idiosyncrasy is an unusual or unpredictable reaction specific to an individual’s genetics.
Allergy is an immune-mediated hypersensitivity reaction to a drug.
Understanding these responses is vital to preventing adverse effects and tailoring personalized treatments.
Pharmacokinetics: The Journey of a Drug in the Body
Once a drug enters the body, it undergoes four main processes collectively known as ADME — Absorption, Distribution, Metabolism, and Excretion.
1. Absorption
Absorption is the process by which a drug passes from its site of administration into the bloodstream. It depends on factors such as membrane permeability, lipid solubility, ionization, and blood flow.
Drugs cross biological membranes via various transport mechanisms — passive diffusion, active transport, and facilitated diffusion.
2. Distribution
After absorption, the drug is distributed to different tissues. Distribution depends on blood flow, plasma protein binding, and tissue affinity. Some drugs remain confined to plasma, while others accumulate in fat or muscle tissues.
3. Metabolism
Drug metabolism, mainly occurring in the liver, transforms drugs into more water-soluble compounds for excretion.
Enzyme induction increases the activity of metabolic enzymes, leading to faster drug clearance.
Enzyme inhibition, on the other hand, slows metabolism and increases drug concentration, potentially causing toxicity.
These interactions are critical in drug-drug combinations and clinical dosing adjustments.
4. Excretion
Finally, drugs and their metabolites are eliminated from the body through kidneys (urine), bile, lungs, or sweat. The kinetics of elimination describe how quickly this process occurs — usually following first-order kinetics, where a constant fraction of the drug is eliminated per unit time.
Kinetics of Elimination and Clinical Relevance
The rate of elimination determines the drug’s half-life (t½) — the time required for its plasma concentration to reduce by half.
A short half-life means frequent dosing.
A long half-life supports sustained or once-daily dosing.
By understanding elimination kinetics, clinicians can design dosing regimens that maintain therapeutic concentrations without reaching toxic levels.