BBC One, Sunday, 19 June 2005 at 22:15 BST
Panorama explores the latest scientific research on the effects of cannabis on the human mind.
In particular, the programme will look at the growing evidence of links between cannabis and psychotic illness in young people.
British youngsters are using Cannabis earlier and smoking more of it than any previous generation. Most don't even think of it as a drug, and the popular perception is that it has no serious long-term effects. However, the truth is that, until very recently, very little was known about how cannabis actually affects developing brains.
The programme meets young people who have developed psychotic illnesses after heavy cannabis use and speaks to the psychiatrists who have found links between users' genetic makeup and the risk of developing mental illness.
The programme features the work of scientists who have used the latest technology to look inside peoples' heads and see - quite literally - how cannabis changes the way people think. It meets the scientist whose unpublished new research suggests that cannabis can cause long-term chemical changes in users' brains and supports the idea that it can be a "gateway" making users more likely to take other drugs.
Reporter: Justin Rowlatt
Producer: Angeli Mehta
Assistant Producers: Steve Barrett, Katy Stead
Deputy Editors: Andrew Bell, Frank Simmonds
Editor: Mike Robinson