Medical trials.

http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/drugs_use/drugsrev1.shtmlDrugs are substances that change chemical reactions in the body. Medical drugs relieve disease and illness, and are extensively tested before being used. Recreational drugs such as alcohol and tobacco are taken by people because they like the effects they have on their bodies, but they may be addictive. Cannabis and heroin are illegal recreational drugs that are very addictive.