štetno

1. harmfully
   Stres štetno utiče na zdravlje. — Stress has a harmful effect on health.
2. (as a predicate) it is harmful
   Štetno je dugo sedeti za kompjuterom. — Sitting at the computer for a long time is harmful.

"Harmful to" is usually po + acc. or za + acc.: štetno po zdravlje.