If your dog is coughing up phlegm with blood, it can be frightening to see and usually signals that something serious is affecting the lungs, throat, or respiratory system. Even small amounts of blood mixed with mucus deserve immediate attention.
We outline the common causes of a dog coughing up phlegm with blood, what you can do, and when to seek veterinary help.
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Dog Coughing Up Phlegm With Blood: Why It Happens
A dog coughing up phlegm with blood is most often reacting to infections, airway irritation, lung disease, trauma, heart problems, or internal inflammation that causes fragile tissues to bleed.
While occasional coughing is normal, the presence of blood means the respiratory system is struggling.
Dog Coughing Up Phlegm With Blood: Common Causes
Respiratory Infections
Advanced respiratory infections can inflame and damage the lining of the lungs and airways. When this tissue becomes irritated, coughing becomes deeper, harsher, and more frequent — sometimes strong enough to rupture tiny blood vessels. This leads to blood-streaked phlegm.
Dogs with severe infections may also have fever, loss of energy, nasal discharge, or rapid breathing.
Inflammation causes mucus buildup, and the dog may cough repeatedly in an effort to clear the airways.
Over time, the constant irritation makes bleeding more likely. Respiratory infections are dangerous because they can progress to fluid in the lungs or severe respiratory distress if not treated quickly.
Read more: Dog Coughing Up White Foamy Mucus (What it could mean)
Heart Disease or Congestive Heart Failure
Heart disease is another major cause of coughing with blood. When the heart cannot pump efficiently, fluid may accumulate around the lungs, creating pressure and irritation that leads to persistent coughing.
Over time, the cough becomes forceful and may bring up mucus mixed with blood.
Dogs with heart disease often cough more at night or after lying down. They may also tire easily, breathe faster, or become restless.
As the disease progresses, coughing episodes become deeper and more productive, increasing the risk of blood appearing in the phlegm. Heart-related coughing is serious because it can signal worsening heart failure that needs immediate attention.
Throat or Airway Injury
Injury to the throat or airway can cause bleeding that mixes with the mucus produced during coughing.
Dogs may injure their throat by chewing sticks, swallowing sharp objects, pulling excessively on collars, or inhaling irritants. A sudden onset of bloody phlegm is especially suspicious for trauma.
Irritated or damaged tissues become extremely sensitive, and even mild coughing may cause bleeding.
Dogs may swallow repeatedly, gag, or act uncomfortable when eating or drinking. Airway injuries require prompt evaluation because swelling or infection can quickly follow.
Foreign Object in the Airway
A small object lodged in the throat or airway can cause coughing fits that bring up mucus and blood. Grass awns, seeds, small toys, or pieces of food can become stuck and cause severe irritation.
Dogs may cough suddenly and repeatedly, gag, paw at their mouth, or appear panicked.
Foreign objects can migrate deeper into the respiratory tract, making the situation life-threatening.
Chronic irritation from even a partially swallowed object can create bleeding that appears in the phlegm. This cause always requires veterinary intervention.
Lung Disease or Internal Growths
Tumors or masses in the lungs, trachea, or throat can cause chronic coughing with mucus that becomes bloody over time. As masses grow, they disrupt airflow, irritate sensitive tissues, and weaken blood vessels.
Dogs may develop louder breathing, wheezing, or difficulty inhaling fully.
Lung disease often presents with gradual symptoms — reduced stamina, weight loss, lethargy, or persistent coughing that worsens in frequency.
When blood appears, it indicates that the irritation has advanced. Tumors inside the respiratory tract require diagnostic testing and careful management.
Chronic Bronchitis or Airway Inflammation
Dogs with chronic bronchitis frequently cough to clear thick mucus from their lungs. Over time, the repeated coughing causes inflammation of the airway lining.
When the irritation becomes severe, small blood vessels may rupture, causing streaks of blood in the phlegm.
Environmental irritants such as smoke, dust, allergens, or cold air can make symptoms worse.
Dogs may cough more after exercise or when excited. Chronic bronchitis becomes serious because ongoing inflammation can permanently damage the airway structure.
Parasites Affecting the Lungs
Some parasites — such as lungworms or heartworms — can invade the lungs and cause coughing, phlegm, and internal bleeding. These parasites irritate lung tissues, create inflammation, and may damage blood vessels.
Affected dogs may cough persistently, lose weight, or show reduced energy levels.
The presence of blood in the mucus signals advanced irritation or vascular damage. Parasitic lung disease is dangerous because it often progresses silently before symptoms appear.
Read more: Dog Coughing with No Other Symptoms (What it means)
What to Do If Your Dog Is Dog Coughing Up Phlegm With Blood
First, stay calm and observe the episode carefully. Do not allow your dog to exercise, as activity can worsen coughing and bleeding. Keep your dog in a quiet environment and note the amount and color of the blood — bright red indicates fresh bleeding, while darker blood suggests deeper irritation.
Avoid giving any over-the-counter cough suppressants or human medications, as these can be harmful.
Offer fresh water but do not force drinking if your dog seems nauseous or distressed. Keep your dog warm and comfortable while monitoring breathing rate and effort.
If coughing episodes are frequent, violent, or worsening, take your dog to a veterinarian as soon as possible. Because coughing up blood is rarely mild, evaluation is necessary even if your dog seems otherwise alert.
When to Call or Visit Your Vet
Seek veterinary care immediately if your dog is coughing up blood-streaked phlegm, especially if you notice:
• Rapid or labored breathing.
• Persistent coughing or worsening episodes.
• Lethargy, weakness, or collapse.
• Pale gums or signs of shock.
• Coughing after choking, trauma, or pulling on a leash.
• Fever, nasal discharge, or noisy breathing.
• Difficulty eating, swallowing, or drinking.
• Sudden coughing in a dog with heart disease or known lung issues.
A veterinarian may need to perform X-rays, bloodwork, airway exams, or heart evaluations to determine the cause and start treatment right away.
Read more: Dog Throwing Up and Coughing (What it means and what to do)
Key Takeaway
A dog coughing up phlegm with blood is usually experiencing lung disease, infection, heart problems, airway irritation, parasites, or injury.
While phlegm alone can be mild, the presence of blood always signals that the respiratory system is inflamed or damaged. Quick action and proper veterinary care can protect your dog’s lungs, ease coughing, and prevent complications — helping your dog breathe more comfortably and recover safely.