A drug is any substance (other than food that provides nutritional support) that causes physiological changes (usually psychological changes) when inhaled, injected, smoked, eaten, absorbed, or dissolved under the tongue through a patch on the skin.

In pharmacology, medicine is a kind of chemical substance with known structure, not a nutrient of essential dietary ingredients. When it is applied to living organisms, it will produce biological effects. Drugs, also known as drugs, are chemicals used to treat, cure, prevent or diagnose diseases or promote health. Traditionally, drugs are extracted from medicinal plants, but recently they are also obtained by organic synthesis. Drugs can be used for a limited period of time or for chronic diseases on a regular basis.

Drugs are generally divided into several categories: those with similar chemical structure, the same mechanism of action (combined with the same biological target), the related mode of action, and the related drug groups used to treat the same disease. The anatomic therapeutic chemistry classification system (ATC) is the most widely used drug classification system, which assigns a unique ATC code to drugs, which is an alphanumeric code that can be assigned to specific drug categories within the ATC system. Another major classification system is biopharmaceutical classification system. This classifies drugs according to their solubility and permeability or absorption characteristics.

Psychoactive drugs are chemicals that affect the function of the central nervous system and change perception, mood or consciousness. They include alcohol, inhibitors (and small amounts of stimulants), as well as nicotine and caffeine stimulants. These three drugs are the most widely used psychotropic drugs in the world and are also considered as recreational drugs because they are for recreational purposes rather than for medical purposes. Other recreational drugs include hallucinogens, opium and amphetamines, some of which are also used in spiritual or religious settings. Some drugs cause addiction, and all drugs have side effects. Overuse of stimulants can promote excitatory psychosis. Many recreational drugs are illegal and international treaties exist for the purpose of banning them, such as the Single Convention on narcotic drugs.

Drugs are used to cure or improve any disease or medical condition. It can also be used as a prophylactic that has future benefits but does not treat any existing or preexisting diseases or symptoms. The government usually divides the distribution of drugs into three categories: over-the-counter drugs, which can be used in pharmacies and supermarkets without special restrictions; over-the-counter drugs without prescription distributed by pharmacists and prescription only drugs that must be prescribed by licensed medical professionals (usually doctors).

In the UK, over-the-counter drugs, known as pharmacy drugs, can only be sold by or under the supervision of a pharmacist at a registered pharmacy. The drugs are labeled with the letter P. The range of drugs available without a prescription varies by country. Drugs are usually produced by pharmaceutical companies and are often patented to give developers the exclusive right to produce them. Drugs that do not have patents (or have expired patents) are called generic drugs because they can be produced by other companies without restrictions or licensing by patent holders.

Drugs are usually classified into drug categories. A group of drugs will have similar chemical structure, or have the same mechanism of action, the same related mode of action or for the same disease or related diseases. The anatomic therapeutic chemistry classification system (ATC) is the most widely used drug classification system, which assigns a unique ATC code to drugs, which is an alphanumeric code that can be assigned to specific drug categories within the ATC system. Another major classification system is biopharmaceutical classification system. They are grouped according to their solubility, permeability or absorption characteristics.

Many countries and regions have many government agencies, which are responsible for controlling and supervising the production and use of drugs and the implementation of various drug laws. The Single Convention on narcotic drugs, an international treaty proposed in 1961, prohibits the use of narcotic drugs other than those used for medical research and treatment. In 1971, a second treaty, the Convention on psychotropic substances, had to be introduced to deal with newer recreational psychoactive and psychedelic drugs.

The legal status of Salvia province varies in many countries, even in the states of the United States. There are also differences in places where legislation is prohibited.

The U.S. Food and Drug Administration (FDA) is a federal agency responsible for monitoring and protecting public health through the regulation of food safety, tobacco products, dietary supplements, prescription and over-the-counter drugs, vaccines, and biopharmaceuticals. Blood transfusion, medical equipment, electromagnetic radiation emission equipment, cosmetics, animal food and veterinary medicine.