Went to dr for possible sinus infection. been dizzy/lightheaded, sinus pressure, headache, ear ache. gave me amoxicillin 875 mg 3xday. been less than 48 hrs and runny nose now. dizzy/sinus pressure more today. how long til it works?

A long time: Amoxicillin is no longer recommended as a first-line treatment for possible bacterial sinus infection. The good news for you is that 80 to 85% of sinus infections are actually viral. And this time of year the vast majority of nasal symptoms are actually related to allergy, specifically ragweed. Odds are, with or without medicine, your symptoms will last approximately 10 to 14 days.

VIRAL INFECTION: Basic assumption is that your doctor gave you treatment for a bacterial infection. Inasmuch as 90 to 95% of upper respiratory infections are viral, there is no answer to your question. If you have a bacterial infection, it may take 3 to 5 days for you to feel well. With no culture of the nose and no pus in the nose, i believe you have a viral infection and think the antibiotic will not work.