How long until prednisone and z-pack stark working for a sinus infection & i start feeling better. it's been 3 weeks i've been sick. just got medicine?

A few days: You should start to feel better after a few days. The medications will not make you feel better over night. Finish the full prescription as directed by your physician.

A few days: Both should make you feel somewhat improved within 3 days. Be sure to take the full course of therapy for both the prednisone and the z-pack. So long as you are improved I would hold off on any other therapy, such as another antibiotic after the z-pack, for at least 10 days. The z-pack continues to work for another 5 days after you are finished.