Fever blisters on lips can be brought on by a variety of external sources, but the underlying cause is the herpes simplex virus (HSV-1). This virus is most readily spread when fever blisters are present, but may be transmitted even if a person is asymptomatic. While the root cause of fever blisters on lips is contracting HSV-1 from a carrier, external sources like sun damage, stress, or physical trauma can cause the virus to reactivate.

HSV-1 typically goes dormant after the symptoms of the initial infection have subsided. When it reactivates, for whatever reason, the result can be fever blisters on lips. Fever blisters and similar lesions, like canker sores, can also form inside the mouth, on the soft palate, or even in the esophagus. The most common location, though, is the edge of the lips where they meet the skin of the face.

One common cause of fever blisters on lips may be damage from the ultraviolet (UV) radiation of the sun. This can be mitigated by wearing sunblock on the lips when outdoors, so that the UV radiation isn't able to cause the virus to reactivate. Other trauma to the lips can also trigger outbreaks of fever blisters on the lips, so simply avoiding cuts or contusions can help.

Though fever blisters, also known as cold sores, aren't an actual symptom of a cold, the HSV-1 virus is often reactivated by a suppressed immune system. This can result in fever blisters on lips during or after a cold, flu, or other illness.

HSV-1 can also be reactivated by a high level of stress. Like fever blisters brought on by illness, this may be difficult to avoid. While there is no cure for the virus, and thus no cure for fever blisters on lips, there are ways to manage both the outbreaks and HSV-1 itself. Vaccines exist that may make it more difficult to contract HSV-1, while there are antiviral medications available that can help reduce the likelihood of outbreaks.

While they may be similar in appearance, canker sores are a form of mouth ulcer and aren't caused by a virus. The root cause of canker sores is unknown, though they can be brought on by many of the same things as fever blisters, such as trauma, stress, or a suppressed immune system. A physician can perform tests to determine whether the HSV-1 virus is present, though the testing generally must be done after the outbreak subsides.