A drug is something that affects your body.
Not all drugs are illegal to possess. Drugs like cannabis, ecstasy and cocaine are illegal. Other drugs which are against the law include speed, heroin, LSD and magic mushrooms.
Some substances aren't illegal but can be used as drugs including alcohol, headache tablets, cigarettes and glue.
Some legal drugs can only be given out by doctors, like tranquilisers. For this kind of drug doctors give you a special letter to take to the chemist's shop. This is called a prescription.
Drugs affect lots of people's lives. Even legal drugs can be dangerous, for example, when people become addicted to them, like alcohol or smoking.