I have had a cold for 10 days, felt better after day7 but worse after that. Went to the clinic on Sunday and the doctor prescribles Biaxin XL (500mg)once a day. He thinks like I do that it is a sinus infection. I also got nasal spray (Apo-Mometasone) But I still feel so stuffed and not better at all. How long does it take before the antibiotic starts to work?