From the Mayo Clinic:

http://www.mayoclinic.com/health/col...silver/AN01682

Question
Colloidal silver: Is it safe or effective?
My dad takes colloidal silver because he says it helps him maintain good health, and he hasn't been sick in years. But is colloidal silver safe, and is there any evidence that it does improve health?

Answer
from Brent A. Bauer, M.D.
Colloidal silver isn't considered safe or effective for any of the health claims manufacturers make. Nor is it an essential mineral, as some sellers of silver products claim.

More at the link.