It is well known that many homoeopathic medicines are ultramolecular—that is, they are diluted to such a degree that not even a single molecule of the original solute is likely to be present. As drug actions are conventionally understood in biochemical terms, homoeopathy presents an enormous intellectual challenge, if not a complete impasse. Many scientists have suggested that the clinical effects of homoeopathic medicines are solely due to the placebo effect. However, there have been rigorous, replicated, double blind, randomised trials showing significant differences between homoeopathic and placebo tablets.
The response to this has been mixed. Some people remain unconvinced by the evidence, claiming that there must be another explanation, such as methodological bias, for the results. Others point out that the evidence is very strong and argue that homoeopathic medicines must work by some, as yet undefined, biophysical mechanism. One possible explanation, currently being investigated, is that during serial dilution the complex interactions between the solvent (water) molecules are permanently altered to retain a “memory” of the original solute material.