efektivan

1. effective
   Ovo je najefektivniji način da naučiš jezik. — This is the most effective way to learn a language.
   Lek je vrlo efektivan protiv bolova. — The medicine is very effective against pain.
2. (of time, rates) actual; effective
   Efektivna kamatna stopa je veća od nominalne. — The effective interest rate is higher than the nominal one.
   Efektivno vreme igre bilo je samo pedeset minuta. — The actual playing time was only fifty minutes.

In the sense "effective, efficient", efikasan is more common.