A drug, or substance, is any chemical compound that can alter the body's functions when consumed. Drugs can be classified into various categories, including stimulants, depressants, and hallucinogens, each affecting the brain and body in different ways. Some drugs are used for medical purposes, such as treating illnesses or managing pain, while others may be used recreationally. Many drugs can lead to dependence or addiction, which can have serious health consequences. The effects of drugs can vary based on factors like dosage, method of use, and individual differences. Understanding the risks associated with drug use is essential for making informed choices.