Toxic substances are those which have the ability to cause damage to body organs. Treatment of several ailments requires the administration of toxic substance that damage a part of some organs, particular kinds of cells, etc. If an ailment is caused due to hyperactivity in an organ it can be cured by causing a mild damage to the organ to restore normal activity.
In such treatments the organs that are being targeted to cure the ailment are called target organs. For example to treat cancer, the organs affected by the cancer would be the target organs. The aim is to damage the portions of the organs which are affected by cancer.
When toxins are administered to destroy specific organs, their action is usually not limited to those organs and instead they also damage other organs which are healthy. This is known as non-target organ toxicity. For example several anti-cancer medication damage organs that produce blood cells and hair.
Whenever research to find a cure for new illnesses is being done, the objective is always to increase target organ toxicity while reducing non-target organ toxicity as much as possible.