General effectiveness:
* Effic-: As in "efficient" or "efficacious".
* Effec-: As in "effective" itself.
* Oper-: As in "operative" or "operable".
Specific effectiveness:
* Auto-: For self-effectiveness, as in "autonomous".
* Bio-: For effectiveness on living things, as in "bioactive".
* Cosmo-: For effectiveness across the universe, as in "cosmological".
* Electro-: For effectiveness using electricity, as in "electrified".
* Photo-: For effectiveness using light, as in "photoactive".
* Thermo-: For effectiveness involving heat, as in "thermoelectric".
The most appropriate prefix depends on the specific meaning you want to convey.