Phlegm in the throat is almost always caused by one of a few common triggers: post-nasal drip, a respiratory infection, acid reflux, smoking, or airborne irritants. Your body produces about a liter of mucus every day as a normal defense mechanism, but you only notice it when production ramps up or the mucus gets thicker than usual. Understanding the specific cause matters because the fix is different for each one.
How Your Body Makes Mucus
Your airways are lined with specialized cells called goblet cells. These cells secrete gel-forming proteins called mucins, which trap dust, bacteria, and other particles before they can reach your lungs. Tiny hair-like structures then sweep the mucus upward toward your throat, where you swallow it without even noticing. This system runs quietly in the background all day.
Phlegm becomes a problem when something disrupts this balance. Either your goblet cells start producing too much mucus, or the mucus gets too thick to clear efficiently, or both. The result is that sticky, heavy feeling in the back of your throat.
Post-Nasal Drip
The most common reason for persistent throat phlegm is post-nasal drip, where mucus from your nose and sinuses drains down the back of your throat instead of flowing forward. This creates that constant urge to clear your throat or swallow. Common causes include hay fever, sinus infections, viral colds, and exposure to cold air. Certain medications can also trigger it.
If you notice the phlegm is worse in the morning, it may be because mucus pools in your throat overnight while you’re lying flat. Seasonal patterns (worse in spring or fall) point toward allergies as the driver.
Acid Reflux You Might Not Feel
Acid reflux doesn’t always announce itself with heartburn. A condition called laryngopharyngeal reflux, sometimes called “silent reflux,” sends small amounts of stomach acid up into the throat. It only takes a tiny amount of acid, along with digestive enzymes like pepsin, to irritate the sensitive tissue there. Your throat responds by ramping up mucus production as a protective barrier.
What makes this tricky is that the reflux also interferes with the normal mechanisms that clear mucus and infections from your throat and sinuses. So you get more mucus and less clearance at the same time. If you notice throat phlegm that’s worse after meals, when lying down, or alongside a hoarse voice, reflux is worth considering.
Infections and Inflammation
Colds, flu, sinus infections, and bronchitis all trigger a surge in mucus production. This is actually your immune system working correctly. Extra mucus helps trap and flush out the virus or bacteria causing the infection. The phlegm usually resolves within one to three weeks as the infection clears.
Chronic conditions produce a different picture. In asthma, the airways produce a type of mucus that sticks tightly to the airway walls and is difficult to dislodge. In chronic bronchitis and COPD, the mucus is more loosely attached but produced in excess, often for months at a time. Both conditions involve changes to the airway lining itself: the goblet cells multiply and the walls thicken, creating a cycle of overproduction and obstruction.
Smoking and Air Quality
Smoking is one of the strongest drivers of excess phlegm. Research published in PLOS ONE found that current smokers had significantly greater goblet cell density and mucin volume compared to nonsmokers, even among smokers who hadn’t yet developed COPD. Active smoking was the single most important predictor of how many mucus-producing cells lined the airways. This effect is dose-dependent: the more you smoke, the more goblet cells your airways develop.
Beyond cigarettes, air pollution, chemical fumes, dust, and strong fragrances can all provoke a mucus response. Your airways treat these irritants the same way they treat a virus: by producing more mucus to trap and expel them. If your phlegm is worse at work or in certain environments, the air itself may be the trigger.
Does Dairy Actually Cause Phlegm?
This is one of the most persistent health beliefs, and the evidence doesn’t support it. Drinking milk does not cause your body to produce more phlegm. What likely fuels the myth is a sensory trick: when milk mixes with saliva, it creates a slightly thick coating in the mouth and throat that feels like mucus but isn’t. A small study in children with asthma found no difference in symptoms whether they drank dairy milk or soy milk. If you feel more phlegmy after dairy, the sensation is real, but the mucus increase is not.
What Phlegm Color Tells You
Clear phlegm is typically produced by allergies and mild irritants. Yellow or green phlegm often signals that your immune system is fighting an infection, as the color comes from enzymes released by white blood cells. Brown phlegm can result from smoking or inhaling pollutants.
However, color is less reliable than most people assume. A study in Clinical Microbiology and Infection found that when patients self-reported their phlegm color, it had only 39% specificity for detecting a bacterial infection. Even when a clinician assessed the color under controlled conditions, specificity only reached 52%. Notably, 78% of mucoid (white or grey) sputum samples still showed bacterial growth. In short, green phlegm doesn’t necessarily mean you need antibiotics, and clear phlegm doesn’t guarantee there’s no infection.
How Hydration Affects Thickness
Staying hydrated is one of the simplest ways to thin out phlegm, and there’s measurable science behind it. A study at the University Hospital of Zurich tested what happened when patients with post-nasal drip drank one liter of water over two hours. Their nasal mucus viscosity dropped from an average of 8.51 to 2.24 pascals per second, a roughly fourfold decrease. About 85% of patients reported that their symptoms improved after hydrating, and none reported worsening.
Warm liquids like tea or broth may offer an additional benefit by loosening mucus through steam. Dry indoor air, especially from heating systems in winter, can thicken mucus and make it harder to clear. A humidifier in your bedroom can help counteract this.
When Phlegm Lasts for Weeks
Phlegm that lingers beyond three or four weeks after a cold, or that shows up without an obvious trigger, often points to one of the chronic causes: allergies, reflux, or an underlying airway condition like asthma or COPD. Blood in phlegm, unexplained weight loss, or difficulty breathing alongside persistent mucus are signs that warrant a medical evaluation promptly. The same applies if you’re a current or former smoker with a new or worsening daily cough producing phlegm, as chronic bronchitis is defined by a productive cough lasting at least three months in two consecutive years.