Homeopathy was pioneered over 200 years ago. Practitioners and patients are convinced it has the power to heal. Today, some of the most famous and influential people in the world, including pop stars, politicians, footballers and even Prince Charles, all use homeopathic remedies. Yet according to traditional science, they are wasting their money.
Skeptic James Randi is so convinced that homeopathy will not work, that he has offered $1m to anyone who can provide convincing evidence of its effects. For the first time in the programme's history, Horizon conducts its own scientific experiment, to try and win his money. If they succeed, they will not only be $1m richer - they will also force scientists to rethink some of their fundamental beliefs.
The basic principle of homeopathy is that like cures like: that an ailment can be cured by small quantities of substances which produce the same symptoms. For example, it is believed that onions, which produce streaming, itchy eyes, can be used to relieve the symptoms of hay fever.
However, many of the ingredients of homeopathic cures are poisonous if taken in large enough quantities. So homeopaths dilute the substances they are using in water or alcohol. This is where scientists become skeptical - because homeopathic solutions are diluted so many times they are unlikely to contain any of the original ingredients at all. Yet many of the people who take homeopathic medicines are convinced that they work. Has science missed something, or could there be a more conventional explanation?