References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
Yes, several STIs can cause mouth ulcers. Herpes simplex (HSV-1) is the most common cause, producing painful blisters that burst into ulcers. Syphilis can cause painless chancres in the mouth. HIV seroconversion sometimes includes mouth ulcers. Knowing the differences helps you recognise when a mouth ulcer might be STI-related.
Here’s what to watch for.
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STIs That Cause Mouth Ulcers
| STI | Typical Ulcer | Pain Level | Timing |
|---|---|---|---|
| Oral herpes (HSV-1) | Cluster of blisters that burst into small ulcers | Painful, burning | Days after exposure; can recur |
| Syphilis (primary) | Single painless firm ulcer | Painless | 10-90 days after exposure |
| Syphilis (secondary) | Mucous patches — grey-white lesions | Usually painless | Weeks to months later |
| HIV (seroconversion) | Small painful ulcers, often with fever | Painful | 2-4 weeks after exposure |
| Gonorrhoea (pharyngeal) | Usually no ulcers; sore throat or asymptomatic | Variable | Days after exposure |
Non-STI Causes of Mouth Ulcers
Most mouth ulcers aren’t caused by STIs. Common causes include:
- Aphthous ulcers (canker sores) — idiopathic, often stress-related
- Trauma (biting the cheek, dental work)
- Vitamin deficiencies (B12, folate, iron)
- Viral infections (hand-foot-and-mouth, chickenpox)
- Crohn’s disease and other inflammatory conditions
- Drug reactions
- Oral cancer (rare, but important to consider with persistent ulcers)
Oral Herpes
Most oral herpes is caused by HSV-1, typically acquired in childhood from kissing or shared drinks. Many people have cold sores from time to time. Features:
- Usually around the lips rather than inside the mouth
- Tingling or burning before blisters appear
- Cluster of small blisters that merge and burst
- Heal in 7-14 days
- Can recur, often triggered by stress, illness or sunlight
Oral HSV-1 can be transmitted to the genitals through oral sex.
Syphilis in the Mouth
Oral syphilis is becoming more commonly recognised:
- Primary: A painless chancre on the lip, tongue or inside the mouth at the site of exposure
- Secondary: Mucous patches — greyish-white lesions, sometimes called “snail track ulcers”
- Both stages are highly infectious
Oral syphilis can be mistaken for other mouth conditions, leading to missed diagnosis.
HIV Seroconversion Mouth Ulcers
Painful small mouth ulcers are one feature of the seroconversion illness that many people experience 2-4 weeks after HIV infection. They usually occur alongside other symptoms like fever, sore throat and rash.
When to Seek Help
- A mouth ulcer that doesn’t heal in 2-3 weeks
- A painless ulcer, especially after a new sexual partner
- Multiple ulcers with fever or general illness
- Any recurring or unusual mouth lesions
- A sore throat plus mouth ulcers plus fever after a new partner
Read more: Mouth problems and HIV | Cold sores vs syphilis | What is herpes?
Frequently Asked Questions
How do I tell a canker sore from an STI ulcer?
Canker sores are inside the mouth, typically small and painful. Syphilis chancres are painless, and herpes usually shows as blisters first. Timing and sexual history matter.
Can chlamydia cause mouth ulcers?
No. Chlamydia can infect the throat but doesn’t typically cause mouth ulcers.
Should I test for STIs if I have a persistent mouth ulcer?
Yes, particularly if it’s painless or follows a new sexual partner. Syphilis testing is quick and important.
Related reading
- Abnormal Genital Discharge and Chlamydia
- Asymptomatic STIs: Silent Infections Explained
- Causes of Painful Sex and How to Treat It
- Cold Sores vs Syphilis Sores: How to Tell the Difference
- Common STI Symptoms in Women
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
