učinkovito

1. effectively; efficiently
   Ovaj lek učinkovito smanjuje bol. — This medicine effectively reduces pain.
   Moramo učinkovitije koristiti vreme. — We need to use our time more efficiently.

The word is primarily Croatian usage; in Serbia it is perceived as a Croatianism and is rare (mostly in translated or official texts). Standard Serbian uses "efikasno" or "delotvorno" — in the examples above a Serbian speaker would normally use those.