TL;DR
Oral ulcers — including aphthous-type canker sores — were documented as a symptom of COVID-19 infection in multiple studies and case reports, particularly during the acute illness phase. The mechanism isn't a direct viral effect on the oral mucosa; it's indirect: COVID causes systemic immune dysregulation, intense physiological stress, potential nutritional depletion (especially zinc and B12), and medication side effects (particularly from treatments affecting gut absorption). Post-COVID (long COVID) patients also report higher rates of oral symptoms including ulcers. Treatment is the same as for any canker sore outbreak — but addressing the COVID-associated drivers (zinc, nutritional recovery) speeds resolution.
Did COVID Cause a Spike in Canker Sores?
Yes, with evidence. Multiple case series and systematic reviews published during and after the pandemic documented oral manifestations of COVID-19, with aphthous-type ulcers among the most commonly reported.
A systematic review by Dziedzic & Wojtyczka (2021 — PMID: 33432827) found oral mucosal lesions in a significant percentage of hospitalized COVID patients. Aphthous ulcers, herpetiform-like clusters, and erosive lesions were documented. Prevalence estimates across studies varied widely (2–30% of COVID patients with oral manifestations) depending on methodology, disease severity, and how systematically oral examination was performed.
Important caveat: Many early COVID patients were also under extreme stress, on multiple medications, and nutritionally compromised — all of which independently cause canker sores. Attributing oral ulcers solely to direct viral effects versus downstream consequences is not straightforward. What's clear is that COVID infection significantly increases the probability of a canker sore outbreak in susceptible individuals.
Why COVID Triggers Canker Sores: The Mechanisms
1. Systemic Immune Dysregulation
COVID-19 produces a profound immune response — in severe cases, a cytokine storm with elevated TNF-α, IL-6, and IL-1β. These are precisely the inflammatory cytokines elevated locally at aphthous ulcer sites. COVID doesn't just dysregulate immune activity systemically; it creates the same inflammatory environment that drives canker sores at a systemic level, lowering the threshold for mucosal immune attacks.
The same immune remodeling that makes COVID dangerous — T-cell exhaustion, altered regulatory T-cell function, cytokine dysregulation — disrupts the mucosal immune homeostasis that normally prevents aphthous ulcers.
2. Intense Physiological Stress
COVID illness, even mild, represents a significant physiological stress event: fever, hypoxia in some cases, immune activation, sleep disruption, and systemic inflammation. This activates the HPA axis, elevates cortisol, suppresses secretory IgA, and lowers the mucosal barrier — the same mechanism by which psychological stress triggers canker sores, operating at much higher intensity. See Can Stress Cause Canker Sores? for the mechanism in detail.
3. Zinc Depletion
Loss of taste and smell in COVID-19 is mediated in part by zinc — COVID depletes zinc through inflammatory consumption and altered zinc metabolism. Zinc deficiency is one of the most evidence-backed canker sore drivers (serum zinc consistently lower in RAS patients vs. controls). A COVID patient who was borderline zinc-sufficient before infection may become frankly deficient during illness, directly increasing canker sore susceptibility.
4. Nutritional Deficiency During Illness
Acute COVID illness often involves reduced appetite, nausea, and GI symptoms — particularly with the GI-predominant strains. Even a week of significantly reduced intake can affect B12, zinc, and iron status in people already near the lower end of normal. Recovery from severe illness also increases nutritional demands.
5. Medication Side Effects
Several medications used in COVID management affect canker sore risk:
Systemic steroids (dexamethasone): Used for severe COVID. Systemic steroid use can paradoxically trigger canker sore outbreaks in some patients by broadly suppressing immune regulation — the same reason long-term systemic steroid use in other conditions is sometimes associated with oral ulceration.
Antibiotics (for secondary bacterial infections): Broad-spectrum antibiotics disrupt gut microbiome, which can impair B12 and other nutrient absorption and alter systemic immune activity.
Metformin: Pre-diabetic and diabetic patients may be on metformin, which depletes B12 over time. COVID illness in this population stresses a system already nutritionally compromised.
Long COVID and Canker Sores
Long COVID (post-acute sequelae of SARS-CoV-2, PASC) involves persistent symptoms weeks to months after acute infection. Oral symptoms — including ulcers — are among the documented long COVID manifestations, likely reflecting:
- Persistent immune dysregulation: Long COVID involves ongoing cytokine dysregulation and T-cell abnormalities that parallel the mechanisms of RAS
- Autonomic dysfunction: Long COVID affects the autonomic nervous system; the HPA axis and stress response remain dysregulated
- Sustained nutritional compromise: Prolonged reduced appetite, taste changes, and GI symptoms in long COVID maintain the nutritional vulnerability from acute illness
For patients with new-onset or worsening canker sores after COVID infection, the approach is the same as for any new canker sore pattern: test B12, ferritin, zinc, and folate; switch to SLS-free toothpaste; and address the nutritional drivers identified.
COVID Vaccines and Canker Sores
Some patients reported canker sore outbreaks in the days following COVID vaccination, particularly after the mRNA vaccines. This is consistent with the known mechanism: vaccines provoke an immune response — that's the point — and the acute immune activation following vaccination can trigger canker sores in susceptible individuals the same way that immune activation from any infection or stress event can.
This is not a vaccine-specific effect or a concerning adverse event. It's the same mechanism that causes canker sores after any immune-stimulating event, including non-COVID vaccines and common respiratory infections. It does not indicate immune damage or vaccine harm.
Treatment During and After COVID
Treatment of COVID-associated canker sores follows the same approach as any outbreak:
During acute illness:
- Salt water rinse is safe and appropriate
- Barrier patches for pain management — allow eating without severe discomfort
- Maintain hydration and caloric intake despite mouth pain; dehydration during COVID illness is the more immediate concern
Recovery phase:
- Test zinc specifically — COVID zinc depletion is the most COVID-specific nutritional driver, and zinc supplementation in the recovery phase is appropriate if levels are low
- Test B12 and ferritin — particularly if appetite was significantly reduced during illness or if antibiotics were used
- Switch to SLS-free toothpaste if not already done — reduces the ongoing mucosal irritant load during a period of heightened susceptibility