Sinusitis can be caused by several underlying issues including allergens, viruses, fungi, and bacteria. The symptoms can be similar even though the cause can differ: all four cause inflammation within the sinuses, and can cause nasal discharge, facial pain/pressure, and different types of discomfort. However, the treatment will change depending on what’s causing the symptoms.
Antibiotics are not recommended for every sinus infection. They will work only on bacterial infections—not on allergies, fungal infections or viruses. (That’s why when you have a common cold, you don’t get put on antibiotics: colds are caused by viruses, which don’t respond to antibiotics.) Even for bacterial chronic sinusitis, not all antibiotics are effective, and there are conflicting medical data on how long a course of antibiotics a person should take. Doctors and patients are increasingly aware that we need to be careful about prescribing and taking antibiotics, as the more they’re used, the less effective they become, and the greater the chance that antibiotic-resistant “super-bugs” will emerge. The Centers for Disease Control and Prevention, as well as other authorities, say it is very important that people not take antibiotics unless it is necessary, so that these medicines remain effective and antibiotic-resistant bacteria don’t spread.
So, the short answer is: It depends (but follow your doctor’s advice).
How do you figure out which type of sinus infection you have?
Only your doctor can answer that, but the information below may shed some light.
1) How long has it been around?
If a sinus infection has been around for more than a few weeks, it may have gone from “acute”—that is, a one-off event—to “chronic,” which means it sticks around, or “acute recurrent,” which means it comes back several times in a year.
If it keeps coming back, does not go away after treatment, or has stayed around for more than seven days, it might be time to see your doctor. See a doctor right away if you have a high fever, swelling or redness around your eyes, a severe headache, confusion, double vision or other changes to your eyesight, or a stiff neck.
2) What does your doctor’s office exam reveal?
When you arrive for your appointment, your doctor will examine inside your nose and check for tenderness in your face. You may also be given other examinations, such as a nasal endoscopy (a thin tube inserted into the nostril with a camera on the end), CT scan or MRI, or allergy tests. These are used to narrow down the cause as much as possible before prescribing treatment.
3) Has your doctor cultured your sinuses?
Antibiotics target specific types of bacteria. Certain antibiotics are useful for certain kinds of bacteria, while other antibiotics will not have any effect on the bacteria—it can be like trying to use a hammer to screw in a screw, except you can’t even use the hammer to pound in the screw. Culturing the sinuses allows the physician to be able to find out what kind of antibiotic is going to be most effective in treating the bacterial infection—should you use a Phillips head or a flat head screwdriver?
4) Have you followed your doctor’s advice?
It can be difficult to pin down the precise cause of a sinus infection. That is why antibiotics are only available with a doctor’s prescription. And your doctor is the appropriate person to figure out the cause of your sinus infection. Take the full course of antibiotics as directed by your doctor—even if symptoms have gone; otherwise the infection might come back if you stop taking antibiotics too early.
 Mayo Clin Proc. 2011;86(2):156-167