trezven

1. (of thinking, a person) sober; level-headed; rational
   Potrebna nam je trezvena procena situacije. — We need a sober assessment of the situation.
   On je trezven čovek i ne donosi nagle odluke. — He's a level-headed man and doesn't make rash decisions.
2. sober
   Vozač je bio potpuno trezven. — The driver was completely sober.

For "not drunk", trezan is more common in everyday speech.