List of discredited substances

This is a list of substances or materials generally considered discredited.

A substance can be discredited in one of three ways:

1. It was widely believed to exist at one time but no longer is. Such substances are often part of an obsolete scientific theory.
2. It was once believed to have drastically different properties from those accepted now. It was widely claimed and believed to possess significant properties that are no longer attributed to it.
3. It is currently believed to exist as part of a theory that has not met the theoretical and experimental requirements of mainstream science. In particular, such a theory must be predictive.