Common causes of pus in the lips can include an infection from a lip piercing, a cold sore infection tied to the Type I herpes simplex virus, or a condition called cheilitis that can result from certain vitamin deficiencies. Pus in the lips is a sign that the body is actively fighting an infection that can range from mild to serious. Colors of pus range from white to pale yellow, and it forms as a result of immune cell decay during the body's natural response to harmful bacteria. Blisters containing pus usually indicate an infection that can be spread from one person to the next, so dermatologists often advise sufferers to take careful measures for preventing this problem.

Symptoms of pus in the lips often include swelling and pain, and this issue is especially common in lip piercings that have not been kept clean. The lips are particularly sensitive areas of the body for piercing due to their frequent contact with food and other objects. Professional piercers recommend that a new lip piercing is cleaned carefully with antibacterial liquid soap after every meal and any time after the wearer touches it. Signs of a bacterial invasion at the piercing site include pus in the lips along with redness and discomfort. Minor lip piercing infections often clear up on their own, although more serious ones often need treatment with antibiotics.

Cold sores result from a common herpes virus, and they can be additional causes of pus in the lips in some cases. This type of blister normally forms on the upper or lower lip and can be quite uncomfortable. While sufferers can be tempted to pop or lance a cold sore, doing so can easily introduce bacteria into the sore and make the initial problem worse. Pus in an infected cold sore indicates a more serious bacterial infection that normally takes longer to clear up and can sometimes leave a post-inflammatory scar.

Cracks or small blisters at the corners of the mouth are often a sign of cheilitis, a common problem resulting from too little vitamin B12 or iron in the diet. Picking or scratching at these sores can also frequently trigger a bacterial infection that can spread and lead to pus in the lips. Just as with a cold sore, physicians advise that a cheilitis sore is kept clean and left alone as much as possible to facilitate the healing process.