Canker sores vs cold sores: what’s the difference?

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"Mouth ulcer" and "cold sore" get used interchangeably, but they are two different things with different causes, and knowing which you have changes how you treat it and whether it can spread to others. Here is a clear, side-by-side guide to telling canker sores from cold sores, what tends to trigger each, and the signs that mean a sore is worth getting checked.

Canker sores: inside the mouth

Canker sores, or aphthous ulcers, appear on the soft tissues inside the mouth, the inner cheeks and lips, the tongue, or the base of the gums. They look like round or oval ulcers, pale or yellowish in the middle with a red border, and they can sting, especially with spicy or acidic food. They are not caused by a virus and cannot be passed to anyone. Common triggers include a bite or a scratch from a sharp tooth or brace, stress, certain foods, hormonal changes, and deficiencies such as iron, folate or vitamin B12. Our guide to mouth ulcers goes into these in more depth.

Cold sores: on or around the lips

Cold sores are small, fluid-filled blisters that usually form on or just around the lips, on the outside, often starting with a tingling or burning feeling before they appear. They are caused by the herpes simplex virus, which stays in the body and can reactivate, commonly with sun exposure, illness, tiredness or stress. Unlike canker sores, cold sores are contagious, particularly while a blister is weeping, so close contact and sharing cups or utensils is best avoided at that stage.

Telling them apart at a glance

The fastest distinction is where the sore is. Inside the mouth, on the soft lining, almost always means a canker sore. On the outer edge of the lip, as a blister or cluster, points to a cold sore. The next clue is appearance: an open ulcer with a red rim versus a fluid-filled blister that later crusts. And the third is contagiousness, canker sores cannot spread, cold sores can. When any of those is unclear, a dentist can confirm it quickly.

What helps, and what to avoid

Most canker sores heal on their own within one to two weeks; gentle rinses, protective gels and steering clear of spicy, acidic or sharp foods can ease the soreness in the meantime. Cold sores also tend to settle by themselves, and antiviral creams work best if started at the first tingle. Because treatments differ, being sure which you have matters. If a sharp or broken tooth keeps catching the same spot and causing ulcers, that is worth fixing, see our note on mouth discomfort and soreness for related causes.

When to get a sore checked

See a dentist or doctor if a mouth sore lasts more than two to three weeks, is unusually large or painful, keeps coming back, stops you eating or drinking, or comes with feeling generally unwell. Importantly, any sore or patch that does not heal should always be checked to rule out other causes, as our guide to early signs of oral cancer explains. At our clinic in Viman Nagar, Pune, this is part of routine general dentistry.

Sources & further reading

NHS, Dental Health · American Dental Association (MouthHealthy) · NIDCR, US National Institute of Dental & Craniofacial Research · Indian Dental Association