In practice, extrapolation of the experimental curve towards the region marked with the red circle would mean low doses of toxicant T were considered significant for the dose-response analysis in determining its toxic effects. Precisely, extrapolation of the curve beyond the marked point, which corresponds to 2.25 mg-Kg-1, may indicate that, toxicant T causes adverse effects at a dosage of less than 2.25 mg-Kg-1. Therefore, extrapolation of the experimental curve would lead to a different interpretation, meaning that toxicity threshold of toxicant T falls below 2.0 mg-Kg-1. This experimental curve starts form the toxicity threshold point going onwards within the region of observed adverse effects.
Ordinarily, experimental curves for dose-response relationships are used to determine toxicity of chemical or biological toxicants in organisms. According to this experimental curve, toxicant T causes observed adverse effects in mice at dosage levels above 2.0 mg-Kg-1. However, the lowest point falls within the circled region, which is referred to as ‘No Observed Adverse Effects Level’ (NOAEL). This means that toxicant T does not cause any acute toxic effects in mice.
In regard to toxicity threshold, toxicity threshold for toxicant T ranges from 2.0 – 3.0 mg-Kg-1. This implies that, the human body can not withstand dosage levels beyond the threshold point. The liver can not detoxify toxicant T when the levels exceed the threshold. Therefore, potential adverse effects can be observed in humans. In most cases, injuries to the liver are not accompanied with clinical manifestations until 50 percent of the liver is damaged, owing to the toxic effects of the concerned toxicant. For instance, the development of liver cirrhosis manifests its clinical effects at advanced stages when fibrous tissues have occupied over 50 percent of the liver.
Ideally, the region marked with the red circle corresponds to the minimum lethal drug dosage of toxicant T. At threshold level some pathogens may be sensitive to the drug, but others may show low sensitivity. However, pathogens showing less sensitivity to the drug can be destroyed after continued exposure to the drug. Therefore, daily drug intake should be designed to ensure there are adequate concentrations of the appropriate drug in the patient’s body fluids; either in the blood or cerebral-spinal fluid.
In practice, therapeutic drugs are recommended for clinical use only when their toxicity threshold in humans exceeds their lethal concentrations in the body of the target pathogen such as bacteria. At threshold levels, the drug (toxicant) does not cause adverse toxic effects on the patients, but it causes toxicity to the target microorganism.
However, it is worth noting that, long-term exposure to any toxicant may lead to adverse toxic effects, primarily when the body of an individual is unable to excrete the toxicant. For instance, toxicant T can cause chronic effects in humans even when its intake dosages remain below 2.0 mg-Kg-1. Some of the possible chronic effects caused by long-tern exposure to toxicants, which are taken in small doses, include organ damage and impairment of the nervous system. For instance, long-term exposure to Aflatoxin, which is contained in contaminated cereals, causes cancer of the liver.
It is also worth noting that, threshold dosage of toxicant T can cause acute effects on people with high sensitivity with respect to their genetic variability. This was the case with Thalidomide drug, which caused fatal embryonic defects in humans although it did not cause adverse effects in experimental animals.