TL;DR
If the sore is on the inside of your lip, the part you have to pull your lip out to see, it's almost certainly a canker sore. If it's on the outside, on the red part of the lip or the skin right next to it, it's much more likely a cold sore. That distinction matters: canker sores aren't contagious and aren't caused by a virus, while cold sores are a herpes infection that responds to antiviral medicine. Cold sores start as a cluster of tiny blisters that crust over; canker sores are a single shallow ulcer from the start. A few other things show up on the lip and get mistaken for either one, including mucoceles (soft, painless bumps on the inner lower lip), injuries from biting, cracked corners of the mouth, and sun damage on the lower lip. For an inner-lip canker sore, an SLS-free toothpaste, a barrier patch, and a prescription steroid paste for bad ones are the main tools. For a cold sore, start an antiviral at the first tingle.
Inside or Outside? The Simplest Rule
Inside the lip = canker sore territory. The inner surface of the lips is soft, moist, movable lining, the same kind of tissue as the inside of your cheeks. That's exactly the kind of tissue canker sores form on, and the inner lip is one of their favorite spots, especially the inner lower lip.
Outside the lip = cold sore territory. Cold sores (herpes labialis) show up on the red part of the lip (the vermilion), along the line where the lip meets the skin, or on the skin around the mouth.
This rule holds in the vast majority of cases. The exception: herpes can occasionally appear inside the mouth, but in healthy people that's uncommon, and it usually shows up on the hard palate or the gums that are attached to the bone, not the soft inner lip (Arduino & Porter, 2008 — PMID: 18197856). People with weakened immune systems are the exception to the exception, and can get herpes almost anywhere in the mouth.
So: if you have to pull your lip out to see it, it's almost certainly a canker sore. If you can see it in the mirror without touching your lip, think cold sore first.
Telling Them Apart Beyond Location
| Canker sore (inside the lip) | Cold sore (outside the lip) | |
|---|---|---|
| How it starts | A sore, tender spot that becomes a shallow ulcer | Tingling or itching, then a cluster of small fluid-filled blisters |
| What it looks like | One round or oval ulcer, yellow-white center, red rim | Blisters that break, ooze, then crust and scab |
| Number | Usually one, sometimes a few separate sores | A cluster in one spot |
| Contagious? | No | Yes, from the first tingle until fully healed |
| Cause | The immune system attacking the lining of the mouth | Herpes simplex virus |
| Common triggers | SLS toothpaste, biting, stress, low B12/iron/folate/zinc | Sun, colds and fevers, stress, periods |
| Antiviral medicine | Doesn't help | Helps, especially started early |
| Heals in | 1–2 weeks | About 1–2 weeks |
The blister test is the most reliable. If you see blisters, it's not a canker sore. Canker sores never go through a blister stage.
For the full comparison, see Canker Sore vs. Cold Sore.
Other Things That Show Up on the Lip
Not every lip sore is one of those two.
Mucocele
A soft, round, painless bump on the inside of the lower lip, often slightly bluish or the same color as the surrounding lip. It's a small saliva gland that got damaged (usually from biting) and is leaking into the tissue. The lower lip is by far the most common place for them, accounting for about two-thirds of cases in a study of nearly 2,000 (PMID: 36287502).
The difference from a canker sore: a mucocele is a smooth, fluid-filled bump, not an open ulcer, and it usually doesn't hurt. Some pop and come back. If one keeps returning or bothers you, a dentist can remove it with a minor procedure.
A bite injury
Biting your lip, especially if you've just had a numbing shot at the dentist, can leave a raw ulcer that looks a lot like a canker sore. Kids are especially prone to chewing a numb lip after dental work. These heal on their own in a week or two, as long as you stop biting the spot. In someone who's prone to canker sores, a bite can also kick off an actual canker sore at the same spot. See Canker Sores After Dental Work.
Cracked corners of the mouth (angular cheilitis)
Red, sore, cracked skin at the corners of the mouth. It's often caused by saliva pooling in the corners, sometimes with a yeast or bacterial infection, and it's also linked to iron and B-vitamin deficiencies. It's not a canker sore, but because those deficiencies overlap with canker sore triggers, it's a useful clue if you have both. See Iron Deficiency and Canker Sores.
Sun damage on the lower lip (actinic cheilitis)
Years of sun exposure can damage the lower lip, making it dry, scaly, rough, or blotchy, with the border between lip and skin becoming blurry. It's most common in fair-skinned people who spend a lot of time outdoors. Actinic cheilitis is considered precancerous, and lip cancer almost always appears on the lower lip.
A sore on the outer lower lip that crusts, bleeds, heals, and comes back in the same spot, or that never quite heals, isn't a cold sore pattern. Get it looked at by a dentist or dermatologist. Lip balm with SPF is the simple prevention.
Why the Inner Lip Gets So Many Canker Sores
Toothpaste
When you brush, the foam washes right over the inner lips. Sodium lauryl sulfate (SLS), the detergent in most toothpastes, strips the protective layer off the lining of the mouth. In a small study, people with recurring canker sores had about two-thirds fewer sores after switching to an SLS-free toothpaste (Herlofson & Barkvoll, 1994 — PMID: 7825393). If your sores keep showing up on the inside of your lips, this is the first thing to change. See SLS-Free Toothpaste for Canker Sores.
Hello
Hello Naturally Friendly Toothpaste
SLS-free — removing SLS reduces outbreak frequency in susceptible individuals. Fluoride available. Clean ingredient list, widely available.
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Your teeth
The inner lip rests against the front teeth and gets pushed into them when you chew, talk, and play sports. A chipped tooth, a rough filling, or the edge of braces or a retainer can rub the same spot again and again. See Canker Sores From Braces.
Biting
A lot of people chew or suck on their lower lip without realizing it, especially when concentrating or stressed. If your canker sores keep showing up in the exact same spot on the lower lip, that's a strong hint. See Why Canker Sores Keep Coming Back in the Same Spot.
The tissue itself
The inner lip lining is thin and doesn't have the tough outer layer that the gums or hard palate have. It's exactly the kind of tissue canker sores form on.
Treating a Canker Sore on the Inside of the Lip
The good news about the inner lip is that it's easy to reach, which makes it one of the easier places to treat.
Barrier patches
The inner lip is close to ideal for a dissolving patch. It's fairly flat, you can pull the lip out and dry the spot, and you can hold the lip still while the patch sticks. Once it's on, it covers the sore so food and your teeth don't hit it.
Quantum Health
Canker Cover Dissolvable Patch
Dose: One patch per ulcer; lasts several hours · Dissolvable patch that forms a gel barrier directly over the ulcer. Physical protection mechanism — reduces pain from food, saliva, and tongue contact without anesthetic.
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Numbing liquid or gel
For a quick numb before a meal, a benzocaine product works within a minute or two but wears off as saliva washes it away. A liquid that forms a film, like Kanka, tends to stay put a little longer and covers irregular sores better than a gel.
Kanka
Kanka Mouth Pain Liquid
Dose: Apply small amount to ulcer as needed · Benzocaine anesthetic in a film-forming base that adheres better and lasts longer than plain gel. Good choice for pain relief during meals.
Affiliate link
Orajel
Orajel 3X for Mouth Sores Maximum Strength Gel
Dose: Apply sparingly to affected area up to 4x daily · Topical anesthetic. Numbs pain within minutes. Does not speed healing — benzocaine has no anti-inflammatory action.
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Benzocaine shouldn't be used in children under 2.
Prescription steroid paste
For a bad sore, or if you get them often, a dentist or doctor can prescribe a steroid paste or gel that sticks to the inner lip and reduces the inflammation. Topical steroids help canker sores heal faster than placebo, and they're one of the most useful prescriptions for this. See How to Get Rid of a Canker Sore Fast.
In-office treatment
Because the inner lip is easy to see and reach, it's one of the easier places for a dentist to treat a sore in the office with Debacterol, a chemical that seals the surface of the sore and usually takes the pain away quickly.
Need a prescription steroid paste or in-office treatment for a lip canker sore? We'll connect you with a dentist or oral medicine specialist in your area.
Get connected with local help →Leave it alone
It's hard not to keep pulling your lip down to look at it. Every time you do, you stretch and dry the sore. Check it once a day at most.
If It's a Cold Sore on the Outside of the Lip
Start an antiviral early
Antiviral medicine works best started at the first tingle, before blisters appear. A single day of valacyclovir (2 grams twice, 12 hours apart) shortened cold sore episodes by about a day compared with placebo (Spruance et al., 2003 — PMID: 12604544). That's a prescription. If you get cold sores often, ask your doctor for a prescription to keep at home so you can start it as soon as you feel the tingle.
Over-the-counter docosanol cream (Abreva) can shorten a cold sore slightly if it's started early. See Abreva for Canker Sores for why it doesn't work on canker sores.
Steroids on cold sores
A steroid cream on its own does nothing against the herpes virus. The one steroid product approved for cold sores (Xerese) combines a mild steroid with the antiviral acyclovir, and in a trial it reduced the number of cold sores that progressed to full blisters compared with placebo (Evans et al., 2002 — PMID: 12019102). Don't use a leftover steroid paste meant for canker sores on a cold sore, since it won't treat the infection.
Don't use canker sore remedies
Canker sore patches and pastes are designed for the wet lining inside the mouth and don't do anything for a viral blister on the outside of the lip.
Avoid spreading it
Cold sores are contagious from the first tingle until they've fully healed. Don't kiss anyone, share drinks, utensils, lip balm, or towels, or touch the sore and then your eyes. Be especially careful around babies, people with eczema, and people with weakened immune systems.
When to See Someone About a Lip Sore
- A sore inside the lip that hasn't clearly improved after three weeks
- A sore on the outer lower lip that keeps coming back in the same spot, or never fully heals
- A rough, scaly, or blotchy patch on the lower lip that doesn't go away
- A sore with a hard or raised edge
- Frequent cold sores, so you can get an antiviral prescription to keep on hand
- Frequent canker sores, so you can get your B12, iron, folate, and zinc checked
- Lip sores with fever, swollen glands, or sores in other places
See When to See a Doctor for a Canker Sore for more.