Many drug preparations used in anthroposophic medicine are ultra-diluted substances, similar to those used in homeopathy. Homeopathic remedies are not medically effective and are generally considered harmless, except when used as a substitute for a scientifically proven and effective cure. In certain European countries, people with cancer are sometimes prescribed remedies made from specially harvested mistletoe, but research has found no convincing evidence of clinical benefit. Some anthroposophic doctors oppose childhood vaccination, and this has led to preventable outbreaks of disease.