A sore inside your mouth is most likely a canker sore, not a cold sore. The two look different, have different causes, and respond to different treatments. Cold sores are caused by the herpes simplex virus and almost always appear on or around the lips. Canker sores are not viral, not contagious, and form on the soft tissue inside your cheeks, under your tongue, or along your gums. Knowing which one you have determines how to treat it.
Cold Sore or Canker Sore: How to Tell
Cold sores start as fluid-filled blisters that cluster together, usually on the lip border or the skin just outside the mouth. When herpes does appear inside the mouth, it shows up on the hard palate (the roof of your mouth) or the firm tissue of the gums, surfaces that are tougher and more rigid. These intraoral herpes sores also begin as small blisters before breaking open.
Canker sores look different. They’re shallow, round ulcers with a white or yellowish center and a red border. They form on softer surfaces: the inside of your cheeks, the floor of your mouth, the underside of your tongue, or the soft palate toward the back. They are not caused by a virus, they don’t start as blisters, and you can’t spread them to anyone else.
If your sore is a painless or mildly tender white spot on a soft surface inside your mouth, you’re dealing with a canker sore. If it started as a tingling sensation followed by a cluster of tiny blisters on your gums or hard palate, it’s likely intraoral herpes.
Treating a Cold Sore
Cold sores progress through five stages: tingling, blistering, weeping (when the blister breaks open), crusting, and healing. The entire cycle typically takes one to two weeks. Treatment works best when you start during the tingling stage, before blisters form.
Prescription antivirals are the most effective option. Valacyclovir, the most commonly prescribed, can be taken as a single-day treatment: two doses 12 hours apart. Starting it at the first sign of tingling shortens the outbreak and reduces severity. Your doctor or dentist can call in a prescription quickly if you already have a history of cold sores, and some people keep a supply on hand for exactly this purpose.
Over-the-counter options exist but deliver more modest results. Docosanol (sold as Abreva) is the only FDA-approved nonprescription antiviral cream for cold sores. In a large clinical trial, it shortened healing time by about 18 hours compared to a placebo, bringing the median healing time to roughly four days. That’s a real but small difference. It needs to be applied five times a day starting at the first tingle.
For sores inside the mouth specifically, creams and ointments are harder to keep in place. A prescription antiviral taken by mouth is generally more practical than a topical treatment when the sore is on your gums or palate.
Treating a Canker Sore
Most canker sores heal on their own within one to two weeks without any treatment. The goal is pain management while they run their course. Rinsing with warm salt water several times a day can reduce irritation and keep the area clean. Over-the-counter numbing gels containing benzocaine provide temporary pain relief and can make eating more comfortable.
Avoid acidic, spicy, or rough-textured foods that scrape against the sore. Switching to a soft-bristled toothbrush and a toothpaste free of sodium lauryl sulfate (a foaming agent linked to canker sore recurrence in some people) can also help. If you get large or unusually painful canker sores, a dentist can prescribe a steroid mouth rinse or a topical paste to speed healing.
L-Lysine and Other Home Approaches
For cold sores specifically, the amino acid L-lysine has some clinical support. In a six-month trial, participants taking lysine supplements had 2.4 times fewer outbreaks, with milder symptoms and shorter healing times. The catch is dosage matters significantly. Research reviews found that less than 1 gram per day was ineffective, while doses above 3 grams per day improved outcomes. Lysine supplements are widely available, but they work better as prevention for frequent outbreaks than as a treatment once a sore has already appeared.
Ice applied to a cold sore during the tingling stage can reduce discomfort and may slow blister formation. Over-the-counter pain relievers like ibuprofen help with both the pain and the mild inflammation that comes with an active outbreak.
What Triggers Cold Sore Outbreaks
The herpes simplex virus lives permanently in nerve cells after the initial infection and reactivates under certain conditions. The most well-documented triggers are stress, sun exposure, cold weather, illness, and immune suppression. These triggers often work together. One case study described a patient who had been under chronic stress for months without an outbreak, but a sudden spike in dietary arginine (an amino acid) tipped the balance and caused a relapse.
Arginine is found in high amounts in nuts, seeds, chocolate, and certain grains. The virus uses arginine to replicate, which is part of why lysine (which competes with arginine for absorption) may help prevent outbreaks. During periods of stress or when you feel an outbreak coming on, reducing arginine-heavy foods while increasing lysine intake is a reasonable strategy, though not a guarantee.
UV exposure is one of the more controllable triggers. Using a lip balm with SPF 30 or higher, especially during outdoor activities, skiing, or beach trips, reduces the chance of a sun-triggered outbreak.
Preventing Spread to Other Body Parts
During an active cold sore, the virus sheds heavily and can spread to new locations on your own body. Touching the sore and then rubbing your eyes is the most concerning risk. Ocular herpes causes eye pain, redness, light sensitivity, swelling, and watery eyes. In more severe cases, it can damage the cornea and affect vision. If you develop eye irritation during or shortly after a cold sore outbreak, get it evaluated promptly.
Wash your hands thoroughly after any contact with the sore. Avoid kissing, sharing utensils, or sharing towels while blisters are present. The American Dental Association recommends replacing your toothbrush after a cold sore outbreak to minimize the risk of reinfection, particularly for people with weakened immune systems.
When a Mouth Sore Needs Attention
A single canker sore that heals within two weeks is not a concern. But sores that last longer than two weeks, come back frequently, are unusually large, or make it difficult to eat or drink warrant a dental or medical evaluation. The same applies to cold sores that don’t resolve within two weeks or spread beyond the initial area. Frequent cold sore outbreaks (six or more per year) can be managed with daily suppressive antiviral therapy, which your doctor can prescribe as an ongoing low-dose regimen.