The human body must be prepared for a daily onslaught of thousands of chemicals. Some will be food materials that are absorbed and metabolized because cells have membrane channels and enzymes to recognize and deal with these compounds. Other compounds may or may not enter the cells of the body. If they do, then the body can recognize such compounds and must deal with them appropriately to prevent deleterious effects. It is not always successful in this aim and many xenobiotics cause diseases and may be fatal if a threshold dose is exceeded.
Toxicology is the study of the adverse effects of chemicals on organisms, thatis the study of poisons, where a poison can be defined as any substance that causes injury, illness or death. Toxicology covers the study of the adverse effects of chemicals including drugs, chemicals acquired from the environment and toxins, which are defined here as harmful substances produced by other organisms, often derived from microorganisms.
Xenobiotics, literally meaning ‘stranger to life’, are substances that do notoriginate in the body but are pharmacologically, endocrinologically or toxicologically active. Thus, they might be drugs or synthetic chemicals or a substance produced in one organism and introduced into another where they would not normally occur. An example would be a compound produced by a plant and ingested as food. Equally, it might be a compound that has been completely synthesized chemically and have harmful effects, for example a poison or a carcinogen, or it could be beneficial, such as a medicinal drug.
Drugs are xenobiotics that are used to achieve certain effects. For example, paracetamol alleviates headaches and aspirin controls inflammatory responses. Alternatively, a drug may also be a compound with pharmacological activity used for ‘recreational’ purposes or taken by an addict. The body has to deal with these drugs and eventually get rid of them. An overdose may exceed the body’s capacity to detoxify these compounds, with potentially disastrous effects. However, if the body inactivates the drug too quickly, then its effects will be short-lived. These are all considerations that pharmaceutical companies need to address when developing a new drug.