If you have a sore inside your mouth, it’s most likely a canker sore, not a cold sore. This distinction matters because the two have different causes and different treatments. Cold sores are caused by the herpes simplex virus and almost always appear on or around the outer border of the lips. Canker sores show up inside the mouth, on the inner cheeks, lips, or tongue, and they’re not caused by a virus at all. Knowing which one you’re dealing with will point you toward the right treatment.
Cold Sore or Canker Sore: How to Tell
Cold sores appear as clusters of small, fluid-filled blisters, typically on the outside of the mouth near the lip border. Before the blisters show up, you’ll usually feel a tingling, itching, or burning sensation in the spot. According to the National Institute of Dental and Craniofacial Research, cold sores form on the outside of the mouth around the lips, while canker sores develop inside the mouth.
A canker sore looks different: it’s usually a single round sore with a white or yellow center and a red border. It forms on soft tissue inside the mouth, like the inner cheek, the underside of the tongue, or the soft palate. Canker sores are not contagious and are not caused by herpes. They can be triggered by stress, minor mouth injuries (like biting your cheek), acidic foods, or nutritional deficiencies.
In rare cases, cold sores can appear on the hard palate or gums, but this is uncommon in people with healthy immune systems. If you see fluid-filled blisters grouped together inside your mouth, it could be a herpes outbreak, but a single white ulcer almost certainly isn’t.
What to Put on a Canker Sore
If your sore is inside your mouth and matches the canker sore description, over-the-counter oral gels and pastes are your best options. Products containing benzocaine (a topical numbing agent) can temporarily relieve pain when applied directly to the sore. Look for formulations specifically designed for use inside the mouth, since many cold sore creams are labeled for external lip use only and shouldn’t be applied to inner mouth tissue.
Antimicrobial mouth rinses can help keep the area clean and reduce irritation. A simple home remedy is a saltwater rinse: dissolve half a teaspoon of salt in a cup of warm water and swish gently. You can also dab a small amount of milk of magnesia directly on the sore a few times a day, which coats and soothes it. Most canker sores heal on their own within one to two weeks. For larger or especially painful sores, a dentist or doctor can prescribe a stronger topical paste or a medicated mouth rinse.
Treating a Cold Sore on the Lip
If your sore is on or near the outside of your lips, it’s likely a cold sore, and the treatment options are different. Cold sores typically last 5 to 15 days and progress through distinct stages: a tingling sensation, swelling, blister formation, crusting over, and finally healing. Starting treatment during that first tingling stage gives you the best chance of shortening the outbreak.
The most widely available over-the-counter option is a 10% docosanol cream, applied five times a day until the sore heals. It won’t cure the virus, but it can reduce discomfort and may speed healing. A systematic review of topical cold sore treatments found that the three main options (docosanol and two prescription antivirals) shorten pain duration by less than a day compared to doing nothing, so expectations should be realistic. The benefit is modest but real, especially when you start early.
For pain relief specifically, look for products containing lidocaine or benzocaine designed for lip application. One common formulation uses 4% lidocaine applied one to three times daily. These numb the area temporarily but don’t affect healing time. Keep in mind that many of these products warn against applying them directly inside the mouth.
Prescription Antivirals for Cold Sores
If your cold sores are frequent or severe, prescription options can make a bigger difference than topical creams alone. Oral antiviral medication is the most effective treatment for cold sore outbreaks. For an active outbreak, the typical prescription involves a one-day course taken at the very first sign of tingling. Starting it early is critical, as the medication works by slowing viral replication before blisters fully form.
Prescription topical creams are also available. Studies comparing the two main prescription topical antivirals show mixed results: one trial found one slightly superior to the other, while a second trial found no difference. Overall, topical prescription options perform only marginally better than over-the-counter creams. That’s why doctors often recommend the oral medication for people who get frequent or painful outbreaks, as it works from the inside and tends to have a more noticeable effect.
L-Lysine and Natural Options
L-lysine, an amino acid available as a supplement, has some clinical support for cold sore management. A long-term pilot study found that after 12 months of supplementation, participants experienced a 49% reduction in healing time. In a separate controlled study, participants taking 1,000 mg three times daily for six months had fewer outbreaks, less severe symptoms, and faster healing compared to a placebo group. For prevention, 500 to 1,000 mg daily is a reasonable dose. During an active outbreak, some people increase to 3,000 mg per day for the duration. Doses above that can cause nausea and stomach cramps.
Medical-grade honey has also shown promise. In one clinical study of 29 patients, those applying a medical-grade honey formulation three times daily healed in an average of 5.8 days, compared to 10 days with their usual treatments. About 86% of patients healed faster with the honey, and roughly 72% reported less pain and itching. Regular grocery store honey isn’t the same product, though. Medical-grade honey is sterilized and standardized for wound care.
How to Apply Treatments Safely
Cold sores are highly contagious, especially when blisters are open and oozing. Every time you touch the sore to apply a cream or gel, you risk spreading the virus to your fingers and then to other parts of your body or to other people. Wash your hands thoroughly with soap and water before and after every application. If you’re using an ointment rather than a cream, consider using a clean finger cot or disposable glove, then disposing of it immediately afterward.
Keep the affected area clean and dry between applications. Avoid picking at blisters or peeling scabs, which slows healing and increases the risk of bacterial infection. Don’t share lip balm, utensils, towels, or razors during an outbreak. If the sore is near your eyes, or if blisters spread to new areas of your face, that warrants prompt medical attention, as herpes near the eye can cause serious complications.
Signs That Need Medical Attention
Most cold sores and canker sores resolve on their own without complications. But certain situations call for a doctor’s evaluation: sores that haven’t healed after two weeks, blisters that develop near or around the eyes, or outbreaks in someone with a weakened immune system from conditions like HIV, cancer treatment, or immune-suppressing medications. In people with compromised immunity, oral herpes can spread beyond the mouth and become a serious systemic infection. A secondary bacterial infection, where the sore becomes increasingly red, swollen, or produces pus, also warrants a visit.