The Facts
Post-nasal drip happens when mucus from the nose "runs" into the
throat. A "runny nose" occurs when mucus inside the nose drips
out of the nostrils. Post-nasal drip is similar to when you have a runny nose,
except that the mucus runs into the throat instead and irritates it. This can
make it harder to breathe.
Causes
The inside lining of the nose produces large amounts of mucus in response
to an irritation or infection. The common cold typically causes a runny
nose, but it can also be a symptom of allergic rhinitis, which is caused by
allergies. Most allergens (substances that cause an allergic reaction) are airborne,
and therefore are inhaled. These can also cause nasal congestion and drip.
A nose that runs continuously is usually a sign of an inflammation, irritation,
or infection that hasn't yet been treated.