Denshi Jisho — Online Japanese dictionary

効く きく 1: to be effective; to show effect;
2: to do its work; to carry out its function well;
3: to be possible to use
Common word, Godan verb with ku ending, Intransitive verb 

You can also

Look up kanji details for 効く.

Try a Yahoo Jisho search for 効く .
Try a Goo Jisho search for 効く .
Try a Google search for 効く .