Most simple mouth ulcers (canker sores) heal on their own in about 1–2 weeks, but any ulcer that lasts longer than 3 weeks should be checked by a doctor or dentist.

Typical healing time

  • Minor mouth ulcers (the common small ones) usually disappear within 7–14 days without treatment.
  • Tiny “herpetiform” clusters also tend to clear in about 1–2 weeks.
  • Larger “major” ulcers can last several weeks and sometimes even months, and may leave a small scar when they heal.

Think of a minor ulcer like a small scratch inside your mouth: annoying and sore, but your body is already working to close it up over about a week or two.

When it’s not normal

You should get medical or dental advice if:

  1. A mouth ulcer lasts more than 3 weeks.
  1. Ulcers keep coming back very often.
  1. The sore is very large, very painful, or makes it hard to eat or drink.
  1. You also have fever, weight loss, swollen glands, or feel generally unwell.

These can sometimes be clues to other conditions (like vitamin deficiencies, gut disease, autoimmune problems, or, rarely, oral cancer), so a professional check is important if things don’t follow the usual 1–2 week pattern.

What you can do while it heals

While you’re waiting out that 1–2 week window, you can usually ease discomfort and help your mouth stay comfortable:

  • Use over‑the‑counter gels or pastes made for mouth ulcers to numb pain and protect the area.
  • Rinse gently with salt water or an antiseptic mouthwash (alcohol‑free is often more comfortable).
  • Avoid spicy, acidic, or very rough foods that rub or sting the sore (like crisps, hard crusts, citrus).
  • Keep up good oral hygiene with a soft toothbrush so the area stays clean but not irritated.

In more severe or recurrent cases, a doctor or dentist may prescribe steroid tablets, medicated mouthwashes, or gels to calm inflammation and pain.

Quick forum-style recap

“How long do mouth ulcers last?”
Most people on health forums say their usual canker sore is gone in about a week, maybe stretching to two, and they only worry and see a doctor if it sticks around beyond 3 weeks or keeps coming back.

TL;DR: Most mouth ulcers last around 1–2 weeks, but if yours is still there after 3 weeks, is getting worse, or you feel unwell with it, get it checked by a healthcare professional.

Information gathered from public forums or data available on the internet and portrayed here.