Table of Contents

Toxicology

Toxicology is the branch of pharmacology that studies the harmful effects of chemicals, drugs, and poisons on the human body. It explains how toxic substances produce cellular injury, organ damage, and systemic complications. Understanding toxicology is essential for medical students and healthcare professionals to manage poisoning cases effectively and reduce morbidity and mortality.

Basic Concepts in Toxicology

1. Dose–Response Relationship

Toxic effects depend on the dose, route of exposure, and duration.

  • Low dose → Therapeutic effect

  • High dose → Toxic effect

  • Extremely high dose → Lethal effect

2. Types of Toxicity

  • Acute toxicity – Single large exposure

  • Chronic toxicity – Repeated small exposure

  • Local toxicity – At site of contact

  • Systemic toxicity – After absorption into bloodstream

General Mechanism of Toxicity

Explanation:

Toxic substances typically follow this pathway:

  1. Exposure (ingestion, inhalation, skin contact)

  2. Absorption into bloodstream

  3. Distribution to organs

  4. Cellular interaction

  5. Organ damage and clinical symptoms

Common cellular mechanisms include:

  • Enzyme inhibition

  • Oxidative stress

  • Mitochondrial dysfunction

  • Membrane disruption

  • DNA damage

Toxicokinetics (ADME in Toxicology)

Toxicokinetics describes how the body handles toxic substances.

  • Absorption: GI tract, lungs, skin

  • Distribution: Via blood to target organs (brain, liver, kidneys)

  • Metabolism: Mainly in liver (may convert to more toxic metabolites)

  • Excretion: Kidneys (urine), bile, lungs

Some toxins become more dangerous after metabolism (e.g., formation of reactive metabolites).

Common Types of Poisoning in Clinical Practice

1. Organophosphate Poisoning

Organophosphates inhibit acetylcholinesterase, causing excess acetylcholine accumulation.

Clinical Features:

  • Salivation

  • Sweating

  • Bronchospasm

  • Bradycardia

  • Muscle twitching

Management:

  • Atropine

  • Pralidoxime

  • Airway support

2. Paracetamol (Acetaminophen) Toxicity

High doses cause liver injury due to toxic metabolite accumulation.

Stages:

  • Early: Nausea, vomiting

  • Later: Liver failure

Antidote:

  • N-acetylcysteine (NAC)

3. Opioid Toxicity

Overdose leads to respiratory depression.

Classic Triad:

  • Pinpoint pupils

  • Respiratory depression

  • Unconsciousness

Antidote:

  • Naloxone

4. Heavy Metal Poisoning

Examples include lead, mercury, and arsenic.

Features:

  • Neurological symptoms

  • Abdominal pain

  • Anemia

Treatment:

  • Chelating agents (EDTA, dimercaprol)

Antidote Mechanism Overview

Types of Antidotes:

  1. Chemical Antidotes – Neutralize poison (e.g., chelators)

  2. Pharmacological Antidotes – Oppose effect (e.g., naloxone)

  3. Physical Antidotes – Prevent absorption (e.g., activated charcoal)

General Principles of Poison Management

  • Ensure airway, breathing, circulation (ABC approach)

  • Identify toxin if possible

  • Prevent further absorption

  • Enhance elimination

  • Administer specific antidote if available

  • Provide supportive care

Factors Affecting Toxicity

  • Age

  • Body weight

  • Genetic factors

  • Liver and kidney function

  • Drug interactions

  • Route of exposure

Children and elderly individuals are more vulnerable to toxic effects.

Importance of Toxicology in Pharmacology

  • Prevents drug overdose complications

  • Guides safe prescribing

  • Helps in emergency management

  • Reduces medication errors

  • Essential for forensic medicine

Key Takeaways for Medical Students

  • Toxicity is dose-dependent

  • Early recognition saves lives

  • Some drugs form toxic metabolites

  • Antidotes must be administered promptly

  • Supportive care is the cornerstone of treatment