oligodynamic

from The Century Dictionary.

• In physical, produced by extremely small forces, or by small quantities of an active substance.
• Chemically active in extremely small quantity, as an oligodynamic solution of a salt of copper, capable of destroying the lower forms of vegetable or animal life, though containing, it is asserted, not more than one part of copper in many million or even many hundred million parts of water.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective That is active in very small quantities; used especially to describe the sterilizing effect of some heavy metals against bacteria