A drug is any substance that can be used to modify a chemical process or processes in the body, for example to treat an illness, relieve a symptom, enhance a performance or ability, or to alter states of mind. The word "drug" is etymologically derived from the Dutch/Low German word "droog", which means "dry", since in the past, most drugs were dried plant parts.

The term drug lacks precise simple definiton. Substances consumed as foods are not generally considered to be drugs, but the same substances may be consumed for other reasons. For example, many 'foods' contain alcohol or caffeine, which are generally considered to be drugs. The terms medication and pharmaceuticals are frequently applied to substances licensed for medical treatment, presumably to avoid confusion with recreational drug use. However, in popular media, pharmaceutical companies are often called drug companies.

The effects of a particular drug can vary greatly depending on a number of factors:

combination with other drugs or foods
means of intake (ingestion, inhalation, injection, absorption)
the personal condition and circumstances of the subject (user or patient)
the user's expectations or beliefs about the drug (placebo effect)
quality of ingredients

All drug use includes a certain set of risks which must be weighed over the benefits. Along with the potential to treat illness and improve quality of life, they also have side effects which may include dependence, addiction, psychological disorders, physical deterioration or even death. Before taking any drug, one should be well aware of all the risks and side effects. For some drugs such as cannabis, their legal status poses more risk than use of the drug itself, as simple possession alone may lead to imprisonment. Others such as alcohol, tobacco, caffeine and chocolate are so integrated into society that we forget that they are even drugs at all.

Two patterns of distribution, licensed and illegal, are created by laws designed to prevent or punish perceived abuse or to protect the interests of licensed producers, suppliers and users. Laws may be designed also (not least with respect to alcohol and tobacco) to generate government tax revenue. Legislation tends however to limit our ideas about which substances should qualify as drugs. Broader ideas (which might include tea, coffee and saffron) allow perception of other patterns of distribution.

In the United States, medical professionals may obtain drugs from drug companies or pharmacies (which in turn purchase drugs from the drug companies). Pharmacies may also supply a drug directly to patients, authorized by a prescription from a medical professional, if the drug can be safely self-administered. Most drugs are relatively high-cost for patients to purchase directly when first distributed, although health insurance may mitigate some of the cost. When the patent for a drug runs out, a generic drug (some known as simply a "generic") is usually synthesized and released by competing companies, causing the price to drop markedly. Drugs which don't require prescription by a medical professional are known as over-the-counter (OTC) drugs and can be sold in stores without pharmacy association.

Illegal drug use is often termed recreational, but recreational drug use can be quite legal. Recreational use of alcohol, for example, is quite legal in many states and countries. Also, illegal users may claim that their use is medicinal or therapeutic: medical necessity has been used successfully in England as a defence against charges of illegal possession of cannabis.

The quality of a drug supplied illegally can be very unreliable.

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