The German chemist Paracelsus is widely credited with being the ‘father’ of modern toxicology. It was he who coined the famous maxim “The dose makes the poison”, although earlier texts are known. Thus, paracetamol is safe when the recommended dose is taken to relieve pain, but very dangerous in overdose.
To be more specific, forensic toxicology concerns itself with the effects of chemicals, drugs and poisons on humans, especially where those compounds are administered with malicious intent, ie in medico-legal matters. For that purpose samples will be taken from the victim (blood, urine, hair, oral fluids etc) and analysed for the presence of any drugs present. Also important is determining the amount of drug present; in other words the forensic toxicologist has to know what therapeutic drug levels are when an individual is taking a particular drug as prescribed by their doctor. The presence of levels above therapeutic are indicative of an overdose of the drug.
Of course, it is up to the police (and other parties) to determine how the overdose was administered, ie accidentally or deliberately, and by whom if homicide is suspected.
It should be mentioned that the study of forensic toxicology is not new. Throughout the ages doctors have been expected to be familiar with the signs and symptoms of common poisons on the human body, and bioassays in the form of food tasters have been employed by monarchs down the centuries.
However, the first reliable analytical test for poison was developed by Marsh in 1836. Marsh’s test, as it became known, detected arsenic in human body fluids and, it is said, led many a woman to the gallows – arsenic poisoning being a favourite method of disposing of inconvenient husbands etc among the ladies.