Horizon investigates the very latest medical and scientific research into the effects of cannabis on the brain and the body, with the medicinal form of the drug now legal. A&E doctor Javid Abdelmoneim wants to find out whether it will help or harm patients. He meets the young epilepsy patient responsible for changing the law around medicinal cannabis in the UK, and sees the remarkable effect it has on his condition. He visits a medicinal cannabis farm in Denmark to learn how a company known for growing the recreational drug are now producing medicinal cannabis to be exported all over Europe. He also travels to Israel, hoping to find out why they have been using cannabis as a medicine for more than 20 years, and meets scientists studying the safety and effectiveness of cannabis in treating pain.