The following are the main scientific issues that arise in the ongoing debate between skeptics and proponents of homeopathy:
Homeopathic claims contradict established scientific facts
Skeptics consider homeopathy to be lacking any plausible mechanism. They often view homeopathy as a pseudoscientific remnant from the age of alchemy, when important concepts such as molecules and germs were understood poorly or not at all. In the view of modern scientists, the basic interactions of molecules are sufficient to explain all known chemical and biological phenomena, even if many processes are too complex to be understood at this time. This consensus developed during and after the formulation of homeopathy, as a result of discoveries like the size of atoms by Loschmidt in 1865, the synthesis of urea by Friedrich Wöhler in 1828, and advances in understanding many diseases.
The primary criticism of homeopathy by established science is the lack of a chemical mechanism to explain how ultra-dilute solutions can retain an imprint of a molecule that no longer exists in solution. Regardless of whether the dilution medium is water or alcohol, molecular physics does not allow for imprinting of anything in a liquid medium once those molecules have been diluted to near nonexistence.
Critics also reject homeopathic theory as being logically inconsistent. Why should only the properties of the one intended remedy be imprinted during dynamization, and not the properties of all of the impurities in the water, particularly since all of the substances in complex mother tinctures are presumably imprinted? Why should artificial shaking and swirling imprint the water, but not similar processes in nature? Why should the same information be imprinted by dynamization with alcohol or by trituration with sugar, although the properties of these substances are very different from those of water?