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# Principles of clinical toxicology

Why any substance can be a poison at the wrong dose, how poisons injure cells, and the difference between hazard and risk.

Updated Nov 2, 2026

Toxicology studies the harmful effects of chemicals, medicines included. Its core idea is that toxicity depends on dose, route, duration of exposure and the person exposed. The same principles explain a paracetamol overdose, a pesticide poisoning and the long-term effects of a workplace solvent.

## Key points

- The dose makes the poison: almost any substance is harmful at a high enough dose, and the dose–response curve links exposure to effect.
- Hazard is a substance's capacity to cause injury; risk is the expected frequency of harm under real conditions of exposure.
- Acute exposure is a single large or short exposure; chronic exposure is repeated low-level contact over months or years, with different effects.
- Many poisons injure through a toxic metabolite, as when paracetamol overwhelms the liver's protective pathways.
- Poisons centres give expert advice and collect data; Colombia's national toxicology line runs day and night.

## Dose, route and time

Whether a contact becomes a poisoning depends on how much enters the body, by which route (swallowed, inhaled, through the skin), for how long and how often. Children absorb some toxins, such as lead, far better than adults. Age, liver and kidney function, nutrition and other drugs change how a toxin is handled, so the same exposure can be trivial for one person and serious for another.

## Mechanisms of injury

- Exaggerated drug action: opioids depress breathing, sedatives depress the brain.
- Blocking a vital enzyme: organophosphate insecticides inhibit acetylcholinesterase.
- Toxic metabolites: methanol and ethylene glycol become dangerous only after alcohol dehydrogenase converts them.
- Cell and organ damage: overdose of paracetamol causes liver injury; some chemicals damage the kidney or lung days later.

## Hazard and risk

Hazard describes what a chemical can do; risk depends on how people are actually exposed. Risk assessment uses dose–response data from animals and humans to set exposure limits, such as acceptable daily intakes for pesticide residues in food. These limits are reviewed as new evidence appears.

## Delayed toxicity

Some poisons look harmless at first. Liver failure after paracetamol overdose and lung fibrosis after paraquat appear days after ingestion. This is why the history and the time since exposure matter as much as the first examination.

## Poisons centres and toxicovigilance

WHO recommends that every country have a poisons centre: a service that advises on diagnosis and treatment and records cases to spot new hazards (toxicovigilance). In Colombia, the Línea Nacional de Toxicología, funded by the Ministry of Health and the Consejo Colombiano de Seguridad, answers health professionals and the public at any hour and guides reporting to SIVIGILA.

## Memory aid

Toxicity = What (agent) × How much (dose) × How (route) × How long (time) × Who (patient).

## Test yourself

Think of the answer, then tap the question to see it.

## What is the difference between hazard and risk?

Hazard is a substance's capacity to cause harm; risk is the expected frequency of harm given how people are actually exposed.

## Why are methanol and ethylene glycol poisonings sometimes quiet at first?

The parent alcohols are not the main danger; injury comes from metabolites made by alcohol dehydrogenase over the following hours.

## Which two poisonings cause serious organ damage that is delayed by days?

Paracetamol overdose (liver failure) and paraquat ingestion (lung fibrosis).

## What two jobs does a poisons centre do?

It advises on the diagnosis and management of poisoning, and it collects case data to detect and prevent hazards (toxicovigilance).

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## Example drugs

- [Paracetamol](https://durgical.com/en/pharmacology/paracetamol/)
- [Acetylcysteine](https://durgical.com/en/pharmacology/acetylcysteine/)
- [Fomepizole](https://durgical.com/en/pharmacology/fomepizole/)
- [Pralidoxime](https://durgical.com/en/pharmacology/pralidoxime/)
- [Atropine](https://durgical.com/en/pharmacology/atropine/)

## Connections

## Related classes 2

- [Specific antidotes](https://durgical.com/en/pharmacology/classes/specific-antidotes/)
- [Chelating agents](https://durgical.com/en/pharmacology/classes/chelating-agents/)

[← All of pharmacology](https://durgical.com/en/pharmacology/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Chemical safety and health: Prevention and management of cases of poisoning](https://www.who.int/teams/environment-climate-change-and-health/chemical-safety-and-health/incidents-poisonings/prevention-and-management-of-cases-of-poisoning) (World Health Organization, 2026); [Línea nacional de toxicología](https://minsalud.gov.co/salud/prestacion-servicios/Paginas/linea-nacional-de-toxicologia.aspx) (Ministerio de Salud y Protección Social, Colombia, 2026); [Pesticide residues in food](https://www.who.int/news-room/fact-sheets/detail/pesticide-residues-in-food) (World Health Organization, 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/topics/principles-of-clinical-toxicology/