Conditions
Canker sores: why they appear and how to ease them
Canker sores are small mouth ulcers, white or yellow with a red border, that hurt but are not contagious. They can follow a bite, stress, hormone changes or a lack of certain vitamins, and they usually heal on their own in 1 to 3 weeks. If they last longer, keep coming back or come with fever, the doctor sees you by video in Puerto Rico.
Is it an emergency? Call 911.
Is this an emergency?
This is not an emergency service. Call 911 or go to the nearest emergency room if you have:
- Unable to swallow or keep fluids down
- Swelling of the face or neck, or trouble breathing
- Mouth sores with fever, diarrhea, headache or a rash
- A sore that hasn’t healed in 2 to 3 weeks: in-person exam
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At a glance
Canker sores
Common symptoms
- One or more painful red spots that turn into an open sore
- A white or yellow center with a red border
- A small sore, usually under 1 centimeter
- Less often, fever, feeling unwell or swollen glands
- Other names
- Mouth ulcers · Aphthous ulcers
- Usual test
- Blood tests when they keep coming back, to look for low iron, folic acid or vitamin B12
- Treatment
- Home care and pain relief; prescription treatment only when the medical evaluation calls for it
On this page(8)
What are canker sores?
A canker sore is a painful open sore inside the mouth: white or yellow in the center with a red border. According to MedlinePlus, they appear mostly on the inner cheeks and lips, the tongue, the roof of the mouth and the base of the gums. They are usually smaller than 1 centimeter (about a third of an inch).
Canker sores are not contagious and are not the same as cold sores, which appear on the lips and do spread. Nor are they the white patches of oral thrush, which can be scraped off. For a child with fever and sores in the mouth, on the hands or the feet, see hand, foot and mouth disease.
How we treat canker sores by video
You see Dr. Diana Nieves Castro, a Puerto Rico–licensed general practice physician with several years of emergency-department experience, in English or Spanish. With good light and the camera close, she looks at the sores and asks how long you’ve had them, how often you get them, what you eat, what medicines you take, and whether you have fever, diarrhea, a rash or other symptoms.
She tells you how to ease the pain and, if appropriate, prescribes something for the sore and we send your prescription to your pharmacy. If you get canker sores often, she may order blood tests to look for low iron, folic acid or vitamin B12: we give you a lab order for a lab near you. If a sore won’t heal, we refer you to a dentist or a doctor to examine it in person.
This is online urgent care for non-emergencies. We see adults and children of all ages; you need to be physically in Puerto Rico during the visit.
Your doctor
Dr. Diana Nieves Castro, MD
General practice physician
Several years of emergency-department experience.
Founding Executive Director of The Health Initiative.
Your visit in 3 steps
Request a visit online 24/7 — we confirm your time by email or text.
Request your visit.
Book a visit and choose "Doctor visit". You pay by card when you request it.
See the doctor by video.
We send you a secure video link. It works on your phone, tablet or computer. A flashlight helps show the inside of your mouth.
Get your plan.
If appropriate, the doctor prescribes and we send your prescription to your pharmacy. If needed, you get a lab order or an in-person referral.
What a canker sore visit costs
Canker sores are handled in a US$79 doctor visit, the same price for every age, with no membership. You pay by card when you request the visit, and you get a full refund if the visit can’t take place. The lab charges separately. See all prices.
Canker sore symptoms
According to MedlinePlus, the symptoms are:
- One or more painful red spots that turn into an open sore
- A white or yellow center with a red border
- A small sore, usually under 1 centimeter
- Less often, fever, feeling unwell or swollen glands
Why canker sores appear and how to ease them
According to MedlinePlus, they can appear after a mouth injury, such as dental work, brushing too hard or biting your tongue or cheek. They are also linked to stress, low iron, folic acid or vitamin B12, hormone changes and some food allergies. They are more common in women and can run in families. Sometimes no cause is found.
The pain usually eases in 7 to 10 days, and the sore takes 1 to 3 weeks to heal completely; larger ones take longer. In the meantime:
- Avoid hot or spicy foods.
- Rinse with warm salt water.
- You can use over-the-counter mouth-numbing products, as the label directs.
- Use a soft toothbrush and brush gently.
When to see a doctor and when to go in person
According to MedlinePlus, see a doctor if a canker sore doesn’t go away after 2 weeks of home care or gets worse, if you get them more than 2 or 3 times a year, or if they come with fever, diarrhea, headache or a rash.
A mouth sore that hasn’t healed in 2 to 3 weeks needs a dentist or doctor to examine it in person. For your safety, go to the ER or call 911 if you can’t swallow or keep fluids down, there is swelling of the face or neck, or you have trouble breathing.
How we care for you
We evaluate, diagnose and treat; if needed, we give you a lab order for a lab near you.
We see adults and children of all ages. Patients under 21: a parent with patria potestad (parental authority) or a legal guardian is on camera for the whole visit and shows photo ID.
Real-time video visit. We send you a secure video link. It works on your phone, tablet or computer.
If appropriate, the doctor prescribes and we send your prescription to your pharmacy. We do not prescribe controlled substances.
You must be physically in Puerto Rico during the visit (visitors, tourists and cruise passengers are welcome).
In an emergency, call 911.
Frequently asked questions
Are canker sores contagious?
How long do canker sores take to heal?
Why do I keep getting canker sores?
What should I do about a mouth sore that won’t heal?
About this page
Author: The Health Initiative
Medical care provided by Dr. Diana Nieves Castro, MD (PR license 24027)
Disclosure: Founding Executive Director of The Health Initiative
Published: 2026-10-02 · Updated: 2026-10-02
Sources (2)
- Canker sore (MedlinePlus, NIH) (opens in a new tab) (accessed 2026-10-02)
- Oral thrush (MedlinePlus, NIH) (opens in a new tab) (accessed 2026-10-02)