The FDA considers that there is no real concern over the safety of homeopathic products "because they have little or no pharmacologically active ingredients". There have been a few reports of illness associated with the use of homeopathic products, which may be because some homeopathic remedies are prepared by serial dilution of toxic substances, presenting a risk that by accident they might contain undiluted toxic substances. The medical literature contains several case reports of poisoning by heavy metals such as arsenic and mercury  found in homeopathic remedies. However, in cases that they reviewed, the FDA discounted the homeopathic product involved as the cause of the adverse reactions. In one case, arsenic was implicated, although FDA analysis revealed that the concentration of arsenic was too low to cause concern. Perhaps the main concern about the safety of homeopathy arises not from the products themselves, but from the possible withholding of more efficacious treatment, or from misdiagnosis of dangerous conditions by a non-medically qualified homeopath