Colloidal silver is a liquid suspension of microscopic particles of silver.
 The medicinal use of silver
Concentrations of colloidal silver at 5 parts per million or higher have been found to kill numerous infectious bacteria and in the 1930's and before it was widely used by physicians as a mainstream antibiotic.
 The history of silver in healthcare
Produced by pharmaceutical companies under various names, including Protargol and Argyrol, it was replaced by other pharmaceutical drugs including penicillin.
Compounds of silver also have a long history in medicine. Silver nitrate solutions were introduced by Credé in 1880 to protect newborn infants' eyes from infection, but have largely been replaced by antibiotic ointments since 1978. (Silver nitrate solutions are not the same thing as a suspension of colloidal silver.) Silver-containing creams such as silver sulfadiazine have been used in burn centres for more than 100 years.
 How it works
Colloidal silver is reported to kill bacteria by inhibiting the expression of enzymes and other proteins essential to ATP production.
 Contraindications and controversy
Long-term intake of silver products may result in a condition known as argyria, one symptom of which is a blue or gray discoloration of the skin. It occurs when sunlight interacts with silver deposited in the skin, in the same way that silver particles in photographic film darken when exposed to sunlight. It can occur both via ingestion of silver, or through topical application of silver to the skin.
In August 1999 The U.S. Food and Drug Administration (FDA) issued a ruling banning colloidal silver sellers from claiming any therapeutic value for the product. Unless a drug has undergone the rigorous safety and efficacy testing required of pharmaceuticals no medical benefits can be claimed. Such testing has not been conducted with colloidal silver, so the product now has the status of a dietary supplement in the US (dietary supplements cannot claim to cure diseases, only that they "support healthy functioning").
In 2002, the Australian Therapeutic Goods Administration ruled that colloidal silver-containing products were no longer exempted from therapeutic goods legislation and had to meet the requirements of other products covered by this law. A TGA investigation found that "there are no current legitimate uses of colloidal silver and that the Surveillance Section of the TGA be requested to investigate the illegal availability of colloidal silver products because of concerns about their significant toxicity. The reasons for the recommendation were that:
"There is little evidence to support therapeutic claims made for colloidal silver products; the risk to consumers of silver toxicity outweighs the value of trying an unsubstantiated treatment, and bacterial resistance to silver can occur; and efforts should be made to curb the illegal availability of colloidal silver products, which is a significant public health issue."