trezan

1. sober
   Vozi samo onaj ko je potpuno trezan. — Only someone who's completely sober drives.
   Nikad ga nisam video treznog na slavi. — I've never seen him sober at a slava.
2. (of thinking) clear-headed; sober
   O tome treba razmisliti trezne glave. — You need to think about it with a clear head.

The synonym 'trezven' means both 'sober (not drunk)' and 'sober-minded'; in the sense 'level-headed', trezven / trezveno is the more usual choice.