Fever blisters are caused by herpes simplex virus type 1 (HSV-1), a highly common infection carried by roughly 64% of the global population under age 50. Despite the name, fever blisters typically form on the outside of the mouth, along the border of the lips, not inside the mouth itself. If you’re dealing with sores inside your mouth, you may actually have canker sores, which are a completely different condition. Understanding the distinction matters because the causes, triggers, and treatments are not the same.
HSV-1: The Virus Behind Fever Blisters
The human oral cavity is one of the primary entry points for HSV-1. The virus attaches to a protein called nectin-1 found on most cells in the oral lining, which allows it to fuse with cell membranes and begin replicating. This initial infection may cause a noticeable outbreak of sores, or it may produce no symptoms at all. Either way, the virus doesn’t leave your body afterward.
After the first infection, HSV-1 travels along nerve fibers to a cluster of nerve cells near the base of the skull called the trigeminal ganglion. There, it enters a dormant state where it produces no symptoms and no active virus. The viral DNA sits quietly inside neurons, sometimes for months or years, until something triggers it to reactivate. When it does, the virus travels back down the nerve fibers to the skin around the lips, producing a new outbreak.
What Triggers an Outbreak
The virus can reactivate repeatedly throughout your life, and the triggers are both internal and external. The most well-documented ones include psychological stress, anxiety, and depression, all of which alter immune function by affecting key immune cells. Sun exposure is another major trigger. Ultraviolet-B radiation appears to suppress the local immune response in the skin, giving the dormant virus an opening to reactivate.
Other recognized triggers include:
- Physical exhaustion or fatigue
- Fever or illness (which is where the name “fever blister” comes from)
- Hormonal changes, such as those during menstruation
- Local trauma to the lips, including dental procedures
- Immunosuppression from conditions like diabetes or other infections
- Dietary deficiencies
Not every person with HSV-1 gets frequent outbreaks. Some people experience them several times a year, while others rarely or never have visible sores. The reasons for this variation aren’t fully understood, but individual immune response plays a large role.
Fever Blisters vs. Canker Sores
If your sore is inside your mouth, on the inner cheek, inner lip, or tongue, it’s almost certainly a canker sore, not a fever blister. This is the single easiest way to tell them apart. Fever blisters form on the outside of the mouth, generally around the border of the lips. Canker sores form only inside the mouth and appear as white or yellow open sores.
Canker sores are not caused by a virus. They’re not contagious, and their exact cause is unclear, though they’re linked to stress, minor mouth injuries, acidic foods, and certain nutritional deficiencies. Fever blisters, by contrast, are caused by HSV-1, are contagious, and begin as fluid-filled blisters rather than open ulcers.
How Fever Blisters Progress
A typical outbreak runs one to two weeks from the first symptom to full healing, following a predictable pattern. On the first day, you’ll notice tingling, itching, or numbness on your lip. This prodromal stage is your earliest warning. Within 24 hours, small bumps appear, usually three to five of them along the outer edge of the lip. These bumps quickly fill with fluid and the area becomes red, swollen, and painful.
By days two to three, the blisters rupture and ooze clear or slightly yellow fluid. This weeping phase is when you’re most contagious, though you’re contagious from the very first tingle until the sore has completely healed. Over the next day or two, a golden-brown crust forms. The scab typically falls off within six to 14 days, and the skin underneath may look slightly pink for a few more days before returning to normal.
How the Virus Spreads
HSV-1 spreads through direct contact, most commonly through kissing or sharing utensils, lip balm, or towels. What makes it so widespread is that you don’t need to have a visible sore to transmit it. At least 70% of people with HSV-1 shed the virus from their mouth at least once a month without any symptoms. Many shed it more than six times per month. The virus can be present at multiple sites inside the mouth, typically for one to three days at a time, at levels sufficient to infect another person.
This asymptomatic shedding is a major reason why HSV-1 is so common. Most people who carry the virus were infected during childhood through casual contact with a family member.
Reducing Outbreak Frequency and Duration
Antiviral treatments can shorten outbreaks but won’t eliminate the virus. Over-the-counter topical creams containing docosanol (sold as Abreva) reduce healing time by about 18 hours when applied at the first sign of tingling. Prescription-strength topical acyclovir cream shortens outbreaks by roughly half a day. Prescription oral antivirals are more effective: valacyclovir taken at the first sign of an outbreak can cut the episode short by 1.1 to 1.3 days, and famciclovir can reduce healing time from about 6 days to 4 days.
Starting treatment early is the key. The closer to the initial tingle you begin, the better the results.
The Lysine Question
You may have heard that the amino acid lysine can prevent outbreaks. The idea is that HSV-1 needs arginine-rich proteins to replicate, and lysine competes with arginine, potentially slowing viral reproduction. Some controlled studies found that people taking at least 1 gram of lysine daily while eating a low-arginine diet had significantly fewer recurrences. One crossover study found roughly half the number of outbreaks with lysine supplementation (0.89 per period vs. 1.56 on placebo). However, doses below 1 gram per day without dietary changes appear ineffective, and the overall evidence is mixed. Larger, more rigorous trials are limited.
Arginine-rich foods include nuts, chocolate, and seeds. Lysine-rich foods include meat, fish, dairy, and legumes. Some people who get frequent outbreaks find that shifting this dietary balance helps, though it’s not a reliable standalone strategy.
Protecting Other Parts of Your Body
HSV-1 can spread from the mouth to other areas, and two complications are worth knowing about. Touching a cold sore and then rubbing your eyes can transfer the virus to the cornea, causing a condition called HSV keratitis. This can lead to scarring and, if untreated, vision loss. Washing your hands thoroughly before touching your eyes during an outbreak is important.
The virus can also spread to the fingers through a break in the skin, causing painful blisters on the fingertips known as herpetic whitlow. This is most common in people who bite their nails or pick at cold sores. Avoiding direct contact with the blister fluid and keeping the area clean reduces the risk of spreading the infection to new sites.