You’ve probably heard it said that you can tell if you have a virus or a bacterial infection by the color of your mucus. Clear means you just have to wait it out. Greenish yellow means it’s time to head to the doctor for some antibiotics. But is this always the case? No.

Many times, discolored mucus is the result of a sinus infection. The problem is that some sinus infections are viral, and the great majority are caused by a fungus. If your sinus infection is fungal or viral, antibiotics won’t help. You may feel some lessening symptoms if you get bed rest and drink plenty of fluids, but it isn’t the antibiotic that is helping. More likely the antibiotic is what is causing your stomach discomfort. At the same time, your body is building up resistance to the antibiotic. Now, if you actually get a bacterial sinus infection, the same antibiotic may not be able to fix the problem.

So what is up with the green mucus? It’s actually caused by iron, which gives off a greenish color. When you have an infection, your body produces extra white blood cells to fight it off. These cells produce an enzyme which breaks down the bacteria. Iron is found in the enzyme, resulting in the discolored mucus. The longer it hangs out in your sinuses, the more green it will appear when you finally expel it. In fact, early morning mucus can be discolored just because you’ve been sleeping all night and haven’t been able to clear it out. So don’t assume you have a bacterial infection just because your first nose-blowing session in the morning creates a discolored mess.

The only time to really get worried is when your mucus resembles pus when you blow it out or cough it up. Just taking a couple of days off to rest and drink fluids is usually enough to get past sinusitis. Try a nasal decongestant or a sinus rinse to recover more quickly. If your symptoms persist for over a week, then a doctor’s visit is prudent.