A drug is, in the broadest of terms, a chemical substance that has known biological effects on humans or other animals. Foods are generally excluded from this definition, in spite of their physiological effects on animal species. In pharmacology, a drug is “a chemical substance used in the treatment, cure, prevention, or diagnosis of disease or used to otherwise enhance physical or mental well-being.” Pharmaceutical drugs may be used for a limited duration, or on a regular basis for chronic disorders. Recreational drugs are chemical substances that affect the central nervous system, such as opioids or hallucinogens. Alcohol, nicotine, and caffeine are the most widely consumed psychotropic drugs worldwide. They may be used for effects on perception, consciousness, personality, and behavior. Many recreational drugs are also medicinal. Some drugs can cause addiction and habituation and all drugs have side effects. Many drugs are illegal for recreational purposes and international treaties such as the Single Convention on Narcotic Drugs exist for the purpose of legally prohibiting certain substances.