A drug is any substance (with the exception of food and water) which, when taken into the body, alters the body’s function either physically and/or psychologically. Drugs may be legal (e.g. alcohol, caffeine and tobacco) or illegal (e.g. cannabis, ecstasy, cocaine and heroin). Psychoactive drugs affect the central nervous system and alter a person's mood, thinking and behaviour. Psychoactive drugs may be divided into four categories: depressants, stimulants, hallucinogens and 'other'.The effects of drugs will vary from person to person depending on the persons characteristics (such as physical size, gender, mood, diet, fitness, age, expectations and health), the drug itself (such as the amount used and its purity), and how it is taken and the environment a person is in when using the drug. Some people become depressed, angry, aggressive, sleepy, unmotivated, paranoid, anxious or talkative. Drug use can also lead to social and emotional problems and negative effects on relationships with family and friends.People use drugs for a variety of reasons. Some of these include: - To have fun, relax, forget problems or as a form of escapism - To gain confidence and socialise - Out of curiosity - To lessen inhibitions - To remove personal responsibility for decisions - To celebrate or commiserate - To relieve boredom and stress. - Self-medication to cope with problems. Friends, parents, older brothers and sisters and the media can also have some influence over a young person's decision to use drugs.