: Direct tissue injury at the point of application e.g. skin, oropharynx.
: Secondary to absorption into the bloodstream to cause systemic effects/organ damage.
Most drugs or poisons do not have a magical dose at which they will of necessity cause sickness and/or death. While there are statistical LD (lethal dose) 50 levels for many substances, this concept cannot be extrapolated wholesale to predict effect in individuals.
The levels of these substances detected in body fluids and/or tissues at autopsy must be interpreted together with:
the circumstances surrounding the death
the medical history of the deceased
the autopsy findings.
Many factors may influence how a poison actually acts on a person. These include:
1) : Some poisons are dose dependent and will not cause deleterious effects until a certain critical dose is exceeded.
2) : Mainly a feature of corrosives, which if diluted, may have relatively little effect.
3) : If this is slow, the body's detoxification systems may be able to effect some protection from noxious consequences.
4) : A person accustomed to taking a drug, or who has been exposed to low doses of a poison over a period of time may be able to tolerate far more than the usual lethal dose e.g. drug addicts or people on continuous drug therapy such as epileptics. Amphetamines, barbiturates, benzodiazepines and morphine and its derivatives all exhibit this phenomenon.
5) : In general, children and the elderly are more susceptible to poisons. Hepatic detoxification systems are relatively underdeveloped in young children.
6) : Food in the stomach may delay the action of ingested poisons. The lethal dose of a poison by injection is much lower than by oral administration.
7) : A poison in liquid form will be absorbed more quickly from the stomach than the same substance in powder or tablet form. The chemical state of the poison may also be important e.g. two different salts of the same metal may vary in their toxicity.
8) : In general, ill-health is likely to accelerate the effects of a poison. This is especially true if there is dysfunction of the organ primarily responsible for the detoxification of the poison concerned.
9) : Because of hypersensitivity a person may succumb to a much smaller dose of a drug/poison than normal. Penicillin, aspirin, cocaine and heroine fall into this category.
10) : Two compounds may combine to cause a much more lethal effect than the dose of either one by itself would have been expected to do e.g. barbiturates taken with alcohol.
11) : Some poisons may accumulate in the body over long periods until a toxic dose is eventually reached e.g. chronic lead poisoning.
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