A drug is any substance that has a biological effect when taken into the body. Drugs like alcohol and tobacco are legal in the United Kingdom while the possession, use or supply of others such ecstasy, cocaine and cannabis are against the law. Even though cannabis was reclassified from class B to C in January 2004, it is still illegal to posess, supply or manufacture it.

Recent research has looked at the use of drugs in people aged 12-18 years old. Some of the results are shown in the table.

Do all drug users become addicted? In short, the answer is "no" but as always with drugs the real answer is more complicated. There are many different types of drugs and each has its own particular features.

Some drugs like heroin can cause physical addiction. They actually alter the structure of nerve cells in user's brains and this means that their nervous system cannot work properly without the drug. The user is addicted to the drug. If a user stops taking the drug, they will feel withdrawal symptoms as their nerves need to readjust to working without the drug. It may take weeks before their nervous system is back to normal and during this time there is a great temptation to return to using the drug and stop the withdrawal symptoms.

Many drugs can cause psychological dependence. This can be as bad as a physical addiction even though the drug does not cause physical changes in the user's nervous system. Drugs, like solvents, can become a central part to a user's life. They feel depressed and down if they do not use the drug. They crave the feeling that the drug gives them and without it, dependent users cannot face their day-to-day life.

A feature of many drugs is called tolerance. As the user's body adjusts to the drug, a bigger amount needs to be taken each time to get the same feeling as when it was first taken. This can quickly spiral into using more and more of the drug and lead to addiction or dependence.