Methotrexate and mouth ulcers
Why methotrexate causes mouth ulcers and mucositis, what helps, and when ulcers are a sign of toxicity that needs medical attention.
Never start, stop, or change a medication based on what you read here. Bring questions to your dentist, physician, pharmacist, or prescribing clinician.
Quick answer
Mouth ulcers are one of the most common side effects of methotrexate, even at the low doses used for rheumatoid arthritis and psoriasis. They are sometimes mild and self-limiting, sometimes a sign that the dose is too high or that folic acid supplementation is not working. Persistent or severe ulcers should be reported to your prescribing physician promptly. The dental management focuses on comfort, healing, and ruling out infection.
The mechanism
Why methotrexate causes mouth ulcers
Methotrexate works by inhibiting an enzyme that processes folic acid, which is essential for DNA synthesis. The cells most affected are those that divide rapidly, including cancer cells but also the cells lining the mouth, gut, and bone marrow. When the cells lining the mouth cannot replace themselves quickly enough, small areas of the lining break down, leaving painful ulcers.
The ulcers tend to appear in areas of natural friction (the inner cheeks, the tongue edges, the soft palate) and can range from a single small sore to widespread mucositis affecting most of the mouth. Patients on low-dose methotrexate for autoimmune disease usually have intermittent, manageable ulcers. Patients on high-dose methotrexate for cancer can have severe mucositis that affects eating and drinking.
Folic acid supplementation, prescribed routinely alongside methotrexate for autoimmune use, reduces but does not eliminate the risk. Patients who skip their folic acid dose or who do not take the prescribed amount tend to have more ulcers. The ulcers can also signal that methotrexate dosing is too high relative to what your body is clearing, which is why your prescribing physician needs to know about them.
Practical steps
What to do about methotrexate mouth ulcers
Signs to watch for
When to call your dentist
- Mouth ulcers that do not heal within two weeks.
- Ulcers that grow larger over time instead of getting smaller.
- Widespread mouth sores covering large areas of the cheek, tongue, or palate.
- Severe pain that prevents you from eating or drinking adequately.
- Ulcers paired with fever, fatigue, or bruising elsewhere on the body, which could indicate methotrexate toxicity (this also warrants a call to your prescribing physician).
Common questions
What patients ask about Methotrexate and mouth ulcers
KYT Framework
KYT Framework connection
Four questions that shape how Methotrexate and mouth ulcers factor into dental planning.
Structure
Does mouth ulcers change bone, gum tissue, saliva, enamel, or healing support?
Force
Will chewing, grinding, or bite pressure create added risk for vulnerable teeth or healing tissue?
Timing
Is mouth ulcers something to prevent now, monitor, or evaluate soon?
Stability
What plan gives the mouth the best chance to stay stable?
Next steps
What to do about mouth ulcers
The medication side is usually not the right thing to change. The dental side is. Here is where to go next.
Taking Methotrexate and noticing mouth ulcers changes?
Bring your medication list. KYT can evaluate cavity risk, gum health, and treatment timing in person.
Reviewed by Dr. Isaac Sun, DDS · KYT Dental Services · Fountain Valley, CA · Last reviewed: June 2026
This page is general patient education. It does not replace advice from your prescribing clinician, physician, pharmacist, or dentist. Medication information may change; verify with your clinical team.