Generally, the mucous produced in the nose is a response to any kind of inflammation. However there are several other causes as listed below:

• Allergies
  Inflammation in the nose caused from allergies (allergic rhinitis) can cause excessive drainage from the front or back of the nose (post-nasal dip).


• Common cold or the flu
  As a response to infection, the ling of the nose produces mucous in order to help flush out bacteria and viruses


• Cold weather/strong aromas
  It is normal to experience a runny nose during colder weather (called vasomotor rhinitis), and is usually a temporary phenomenon. In some people who are particularly sensitive, this type of runny nose can occur in response to strong perfumes or while eating food.


• Deviated septum
  The nasal septum is the wall that separates the right and left side of the nasal passage of the nose, and consists of cartilage, bone, and mucosa. A deviated septum is defined as a nasal septum that bends, or is displaced, to one or both sides of the nose. This can cause the nasal airway to become narrowed, preventing proper drainage of mucous.


• Cerebrospinal fluid (CSF) leak
  A weakness in the skull base (the part of the skull where the brain sits) can allow a small tear through the lining of the brain, causing thin, watery CSF to leak into the nose. Often times when the head is tilted, this fluid collecting in the nose and sinuses will drain almost like a faucet.


• Pregnancy
  Higher levels of estrogen hormone during pregnancy can cause the nose to become more runny and congested.


• Side effect of medications
  Certain medications, like birth control pills, can cause clear nasal drip.


• Acid reflux
  Although this doesn’t produce nasal drip from the front of the nose, it can many times imitate the sensation of post-nasal drip. When the throat is exposed to acid from the stomach, it produces mucous as a defense mechanism. Over a long period of time, this can lead hoarseness, the feeling of a lump in the throat, or chronic throat clearing.