Metrifonate (INN) or trichlorfon (USAN) is an irreversible organophosphate acetylcholinesterase inhibitor. It is a prodrug which is activated non-enzymatically into the active agent dichlorvos. Metrifonate (INN) or trichlorfon (USAN) is an irreversible organophosphate acetylcholinesterase inhibitor. It is a prodrug which is activated non-enzymatically into the active agent dichlorvos.