Merck Manual

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Pet Owner Version

Factors Affecting the Activity of Poisons


Steve M. Ensley

, DVM, PhD, Department of Anatomy and Physiology, College of Veterinary Medicine, Kansas State University

Reviewed/Revised Oct 2020 | Modified Nov 2022

The consequences of poisoning can depend on more factors than the toxicity of the poison itself. The dose (amount of the compound per unit of body weight) of poison is a primary concern, but the exact amount of poison an animal has been exposed to is seldom known. The number of times the animal is exposed and the length of time over which the exposure has occurred are important. The way in which the animal is exposed affects how much of the poison is absorbed, how it spreads through the body, and possibly how it is metabolized. The condition of the animal at the time of intake of the poison can also be a factor. For example, if the stomach is empty when an animal eats a poisonous substance, vomiting may occur. If the stomach is partly filled, the poison may be retained and lead to toxic effects. Environmental factors, such as temperature and humidity, affect rates of consumption and even whether or not some toxic agents are present. For example, many plant poisons are associated with seasonal or climatic changes, such as winter cold and rainfall.

Different species of animals can react differently to a particular poison because of variations in absorption, metabolism, or elimination. For example, species unable to vomit, such as horses or rabbits, can be poisoned with a lower dose. The age, size, nutritional status, stress level, and overall health of an animal are important factors. In young animals, metabolism is compromised by underdeveloped systems.

The chemical nature of a poison determines its ability to dissolve into different materials (water vs oil, for example). Poisons that dissolve in water spread through the body more easily than those that do not. Substances added to the active ingredient, such as binding agents, outer coatings, and sustained-release preparations, also influence absorption. Generally, as absorption is delayed, toxicity decreases.

Droplet size is an important consideration in sprays and dips, because the dose increases when the droplets are larger. This is one of many reasons to closely follow label instructions and recommended applications. Only formulations intended for animals should be used.

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