Dog Spitting Up Phlegm. When a dog is coughing and spitting up phlegm, there are generally three things that could be causing it. Mucus especially is a dangerous warning;
However, if your dog is throwing up mucus often, a veterinary visit is warranted. Here are five of the most common reasons why dogs throw up bile: A dog coughing up mucus is often affected by a respiratory infection or the kennel cough.