I've been walking wounded (or sittlng-in-comfy-chair wounded) for almost two weeks with a sinus infection that morphed into bronchitis. Early on, I took a week's worth of amoxicillin, the most common antibiotic; it was somewhat bootleg, left over from a family member who fought off an infection quickly and left the lion share of their dose for the next person to get a bug.
Needless to say, it didn't work. That means it was either a viral infection (which doctors can't do much about save treating symptoms) or a Moxy-resistant bacteria. I figured the former until today, when my bronchitis got bad enough to get into my allergist, who doubles as a GP for his allergy patients.
Turns out it was the latter. Moxy has the advantage of being cheap; some places (like the Meijer chain here in the Great Lakes region) will give it away as a loss leader. However, that means that critters have had a chance to develop a tolerance for it; according to my allergist, it only works on about half of modern-day bacterial infections. Bringing out the heavy artillery from the get go might be more expensive, but it heads off a second trip to the doctor when Moxy doesn't hunt and be a better buy over the long haul.
We'll see if the better drugs work.