Don't judge your mucus by its color - Harvard Health Blog - Harvard Health Don't judge your mucus by its color - Harvard Health Blog - Harvard Health Posted: 08 Feb 2016 12:00 AM PST A friend made an offhand comment the other day that caught me off-guard: "When I blow my nose, it's green, so I'm calling my doctor for some antibiotics." I thought this myth had been debunked long ago. In fact, it has been well established that you cannot rely on the color or consistency of nasal discharge to distinguish viral from bacterial sinus infections, or even whether you're dealing with an infection at all. Seasonal allergies are a good example. They can cause all sorts of nasal discharge — thick or thin, yellow, green, or clear — even though there's no infection at all. Why mucus? Before getting into why my friend's nasal discharge might be green, it's worth thinking a bit about why we have mucu...