The following text is taken from NCCAM website .
Homeopathy is a controversial topic in complementary medicine research. A number of the key concepts of homeopathy are not consistent with fundamental concepts of chemistry and physics. For example, it is not possible to explain in scientific terms how a remedy containing little or no active ingredient can have any effect. This, in turn, creates major challenges to rigorous clinical investigation of homeopathic remedies. For example, one cannot confirm that an extremely dilute remedy contains what is listed on the label, or develop objective measures that show effects of extremely dilute remedies in the human body.
Another research challenge is that homeopathic treatments are highly individualized, and there is no uniform prescribing standard for homeopaths. There are hundreds of different homeopathic remedies, which can be prescribed in a variety of different dilutions to treat thousands of symptoms.