Uncoated aspirin tablets, consisting of about 90% acetylsalicylic acid, along with a minor amount of inert fillers and binders. Aspirin is a pharmaceutical<unk>  often used to treat pain, fever, and inflammation. A<unk>  is any chemical substance that causes a change in an organism's physiology or psychology when consumed.<unk> s are typically distinguished from food and substances that provide nutritional support. Consumption of<unk> s can be via inhalation, injection, smoking, ingestion, absorption via a patch on the skin, suppository, or dissolution under the tongue. In pharmacology, a<unk>  is a chemical substance, typically of known structure, which, when administered to a living organism, produces a biological effect. A pharmaceutical<unk> , also called a medication or medicine, is a chemical substance used to treat, cure, prevent, or diagnose a disease or to promote well-being. Traditionally<unk> s were obtained through extraction from medicinal plants, but more recently also by organic synthesis. Pharmaceutical<unk> s may be used for a limited duration, or on a regular basis for chronic disorders. Pharmaceutical<unk> s are often classified into<unk>  classes—groups of related<unk> s that have similar chemical structures, the same mechanism of action (binding to the same biological target), a related mode of action, and that are used to treat the same disease. The Anatomical Therapeutic Chemical Classification System (ATC), the most widely used<unk>  classification system, assigns<unk> s a unique ATC code, which is an alphanumeric code that assigns it to specific<unk>  classes within the ATC system. Another major classification system is the Biopharmaceutics Classification System. This classifies<unk> s according to their solubility and permeability or absorption properties. Psychoactive<unk> s are chemical substances that affect the function of the central nervous system, altering perception, mood or consciousness. These<unk> s are divided into different groups like: stimulants, depressants, antidepressants, anxiolytics, antipsychotics, and hallucinogens. These psychoactive<unk> s have been proven useful in treating wide range of medical conditions including mental disorders around the world. The most widely used<unk> s in the world include caffeine, nicotine and alcohol, which are also considered recreational<unk> s, since they are used for pleasure rather than medicinal purposes. All<unk> s can have potential side effects. Abuse of several psychoactive<unk> s can cause addiction and/or physical dependence. Excessive use of stimulants can promote stimulant psychosis. Many recreational<unk> s are illicit and international treaties such as the Single Convention on Narcotic<unk> s exist for the purpose of their prohibition.
Drug
Biological agent
Allergen
Medication
Corrosive substance
Alcohol 
Persistent organic pollutant
Toxicant