Drugs are substances that cause chemical reactions in the body.
Medical drugs relieve disease and illness, and are extensively tested before being used. Thalidomide one such drug, but it caused unexpected and very damaging damaging side-effects to babies. For instance, many babies were born without one or more arms or legs.
Recreational drugs, such as alcohol and tobacco, are taken by people because they like the effects they have on their bodies; but they are addictive. Heroin and cocaine are illegal recreational drugs that are very addictive.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]