If you use a .9999 pure silver coin like a Canadian Maple Leaf, it won't hurt you but the true effectiveness of colloidal silver comes from silver IONS, and that requires the full processing, using battery-provided electricity and distilled pure H2O.

In the old days farmers used to put a silver coin in their milk urn to keep the bacteria count low, so it does work on that level, but if you have the means to actually create colloidal silver, why bother with a much less effective method?