Mouth and Lips

Oral thrush: why it happens, and why it keeps coming back

The white coating on the tongue and the widespread redness under a denture are two different views of the same yeast

Oral thrush appears when the Candida yeast already living in your mouth loses its balance and multiplies. The familiar picture is a white coating on the tongue and the inside of the cheeks that wipes off, but in denture wearers there is often no whiteness at all: the palate the denture sits on turns red, and the patient takes it for the denture rubbing rather than thrush. Behind thrush that is treated, clears and then comes back in the same place a few weeks later, there is almost always a cause that was never corrected.

Short answer

Thrush in the mouth is an overgrowth of the Candida yeast that is already part of the oral flora. In denture wearers its commonest form is not a white coating on the tongue but widespread redness on the palate the denture sits on; this is called denture stomatitis. Treatment has to be applied to the denture as well as to the mouth, because the yeast lives on the surface of the denture. Sleeping in a denture, a dry mouth, uncontrolled diabetes and a poorly fitting denture are the main reasons it comes back.

The underlying cause: Candida yeast losing its balance and multiplying
Its form in denture wearers: Widespread redness on the palate, possibly with no whiteness
The step most often skipped in treatment: Cleaning and disinfecting the denture itself
The first reason it recurs: Sleeping in the denture, and a poorly fitting denture

White coating, red palate: two faces of the same yeast

Candida is a yeast already present in most healthy mouths. It does not have to come from outside to cause disease; it is enough for the existing balance to be upset. What upsets that balance is well known: a long course of antibiotics, steroid inhalers, less saliva, uncontrolled diabetes, treatments that suppress the immune system and dentures, which offer the yeast a sheltered surface inside the mouth.

The classic thrush picture is white plaques that look like curdled milk on the tongue, the inside of the cheeks and the palate, and that leave a red, tender surface underneath when they are wiped away. A change in taste, a burning feeling in the mouth, discomfort while eating and cracking at the corners of the mouth can go along with it. This is the easiest picture to recognise, but it is not the commonest form in denture wearers.

In denture wearers, thrush most often appears without any whiteness, as redness in the tissue the denture sits on. This is called denture stomatitis. The distinguishing sign is this: the border of the redness matches the border of the denture almost exactly, and the area the denture does not cover looks healthy. There is no way of noticing this picture without taking the denture out and looking in a mirror.

This redness has three grades in clinical use. In this classification, known by Newton's name, the first type is pinpoint redness, with scattered red dots in the tissue. The second type is widespread redness covering the whole area the denture sits on. In the third type the tissue takes on a rough, granular appearance. In the first type, what is mostly responsible is irritation at the points where the denture presses; in the second and third types the picture has more than one cause and is not solved with medicine alone.

The insidious side of denture stomatitis is that it usually causes no pain. The patient feels well, has no complaint, and the picture is only seen once the denture is taken out. That is why it can go unnoticed for years. In someone who does not take the denture out at night, the time the tissue stays exposed to the yeast rises to a full 24 hours; this area, warm, moist and never in contact with oxygen, suits the yeast well.

Studies looking at the characteristics of these patients explain why the picture is so widespread among denture wearers. In one study of patients with denture-related oral thrush, the great majority slept in their dentures, more than half cleaned them poorly and two thirds had reduced saliva. In the same group, a third of the patients had diabetes and they had been wearing a denture for an average of 29 years. In other words, the picture usually arises not from a single cause but from habits stacked on top of one another.

The real reason it comes back is usually this: the yeast lives not only in the tissue of the mouth but also on the acrylic surface of the denture. In the same study, half of the 54 yeasts grown came from the lining of the mouth and half from the surface of the denture; the denture, in other words, was carrying as much as the tissue itself. Treating the mouth with medicine and putting the denture back in as it was means leaving the treatment half done; the redness comes back in the same place soon after the medicine runs out.

The second common cause is a denture that no longer fits. As the bone shrinks over the years, a gap opens up under the denture, the denture moves while you talk and chew, and it irritates the tissue. Irritated tissue is more open to the yeast, and food debris trapped under the denture feeds it. Medicine given without correcting the fit of the denture holds the picture down for a while but does not touch the cause.

Does the appearance point to thrush

The distinction below is not a firm diagnosis but a framework showing when to think of thrush and when to see your dentist or your doctor without waiting.

When it fits

A white coating that wipes off, with red tissue underneathWhite plaques on the tongue, the inside of the cheeks or the palate that lift off when wiped with gauze and leave a red, tender surface behind are the most typical picture of thrush.
The redness stops exactly at the border of the dentureIf the area the denture sits on is red and everything outside it looks normal, this is the most reliable sign of denture stomatitis. You can see it yourself by taking the denture out and looking at your palate in a mirror.
Burning in the mouth, a change in taste, cracking at the corners of the mouthThese three often go with thrush. A crack at the corner of the mouth is usually taken for a separate problem, when it may be the same yeast spilling out of the mouth.
It started after a course of antibiotics or a steroid inhalerA long course of antibiotics and steroid inhalers upset the balance of the oral flora. If the timing fits, there is no need to look far for the cause.

When it doesn't

If the whiteness does not lift off when wipedA white area that stays put and looks stuck to the tissue is not the typical picture of thrush. In that case an examination is needed rather than trying medicines on your own.
If swallowing is painful or difficultThis can show that the thrush is not confined to the mouth. See your doctor without waiting.
If there is no improvement at all within a week of treatmentFailing to respond to the treatment given shows either that the diagnosis is wrong or that there is a cause underneath that has not been corrected; it needs reassessing.
If there is a one-sided, hard-edged area that is growingThrush usually runs a broad, widespread course. A change settled at a single spot, hardening or growing, does not fit that course and must be assessed.

What your dentist checks

The sequence below sets out the assessment steps followed in a patient thought to have oral thrush. The place to go is your dentist or your doctor; this page does not replace that examination.

1
Examination with the denture taken out
The redness on the palate cannot be seen while the denture is in. The first step of the examination is taking the denture out and looking freely at the tissue underneath. Come to the appointment with your denture in; the clinician will take it out as the first step.
2
Does the border of the redness match the denture
Whether the redness is confined to the area the denture sits on is checked. If the borders match, the picture is most likely denture-related, and the treatment has to include the denture.
3
Looking for a trigger
You are asked about the medicines you take, steroid inhalers, any recent course of antibiotics, your blood sugar levels and medicines that dry the mouth. Photographing the boxes and taking them to the appointment speeds this step up.
4
The fit and age of the denture are assessed
Does the denture sit properly, are its edges too long, has a gap opened up underneath it over the years, how old is it: these change the treatment plan. If the fit is poor, relining or replacing it matters as much as the medicine itself.
5
Treatment is applied to the mouth and the denture together
The antifungal your dentist recommends goes hand in hand with brushing the denture every day and taking it out at night to soak in a suitable solution. If either of these two steps is left out, the picture comes back.

What is done, depending on the cause

These do not replace one another; which ones are needed is decided by an examination, and in most patients several are used together.

Antifungal medicine

The gel, drops or mouthwash your dentist or your doctor recommends brings the picture under control. Follow the patient information leaflet for how long and how often to use it; stopping early because the symptoms have gone is one of the known reasons it comes back.

Cleaning the denture and taking it out at night

The denture should be brushed every day and taken out at night to soak in a suitable solution. Leaving the tissue uncovered for a few hours a day is, on its own, the single most effective habit for reducing recurrence, independently of any medicine.

Relining or replacing the denture

A denture with a gap underneath it because the bone has shrunk is relined by refitting its inner surface; a denture that is very old is replaced. While a denture that does not fit is still in place, preventing recurrence with medicine is not possible.

Controlling the trigger

If you use a steroid inhaler, rinse your mouth after every use. If you have diabetes, blood sugar control has a direct effect on whether the thrush comes back. For medicines that cause a dry mouth, speak to your doctor before stopping them.

A fixed alternative where the denture is the source

If thrush keeps appearing in the tissue the denture sits on and the denture can no longer be made to fit by relining or replacing it, options with no base resting on the tissue at all can be discussed. This is the area we work in for patients whose tooth loss has spread to more than one tooth: when the teeth are fixed to implants, there is no removable denture surface covering the palate and staying in the mouth all night. If you have a panoramic X-ray or a CBCT scan to hand, send it through the form; the dentist who will carry out the treatment reads the image and we write to you within 24 hours. This is a preliminary assessment; the final decision is made after an examination and a CBCT scan.

What putting it off changes

Because oral thrush causes no pain, it is easily put off. What putting it off actually costs comes down to these points.

The tissue can turn rough

Redness left untreated for a long time can turn into a granular appearance in the tissue of the palate. Beyond that point, going back with medicine alone becomes harder and remaking the denture comes onto the table.

It spills out to the corners of the mouth

The yeast in the mouth can spill out as cracks and sores at the corners. Putting cream on the crack at the corner without treating the source inside the mouth is the commonest reason that crack keeps coming back.

Eating suffers

Avoiding food because of burning and a change in taste can lead to weight loss and poor nutrition, particularly in older people. A picture dismissed as painless does its harm indirectly here.

The underlying cause is missed

Recurring oral thrush is sometimes the first visible sign of uncontrolled diabetes, reduced saliva or a medicine that dries the mouth. Simply holding the thrush down means silencing that sign too.

How long it takes to clear

The course below is a general framework for a picture where treatment has started and the cause is being corrected too. If the cause stays in place, the question is not how long it takes but how soon it returns.

The first few days
Burning and the change in taste are usually the first symptoms to improve. The redness fades more slowly than these.
One week
If there is no improvement at all within a week of starting treatment, a reassessment is needed; the diagnosis, or a cause that has not been corrected, has to be reviewed.
Two weeks
Most patients with denture stomatitis improve noticeably within two weeks of starting treatment. By this point, denture habits should have changed too.
The following months
The real test comes after treatment: if taking the denture out at night and cleaning it daily are not kept up, the picture comes back in the same place. Every recurrence is a reason to look at the fit of the denture again.

Don't wait if

There is no improvement despite a week of treatment. A picture that is not responding needs reassessing; do not change the medicine on your own.
Swallowing is painful or difficult. This can show that the picture is not confined to the mouth; see your doctor without waiting.
The redness clears and then comes back in the same place. A recurring picture shows that what needs discussing is the cause, not the medicine. The fit of the denture and wearing it at night need reviewing.
A rough, granular appearance has formed on the palate. A change in the tissue may have started; in this picture the denture needs reassessing.

What determines the scope

Treating oral thrush is usually simple, but the scope changes with how far the picture has to do with the denture. These are the items that determine it:

Whether the picture is denture-related: Thrush that clears with medicine alone and denture stomatitis, where the denture has to be dealt with too, are different scopes.
The fit of the denture: There is a clear difference in scope between refitting the inner surface and making the denture again from scratch.
Whether the tissue has changed: If there is settled tissue overgrowth or a granular appearance under the denture, further assessment and a procedure may be needed.
Investigating the underlying cause: Investigating a cause to do with blood sugar, a dry mouth or a medicine falls under your doctor's remit, not your dentist's.

Frequently asked questions

Is the white coating in my mouth thrush?

White plaques that lift off when wiped and leave a red, tender surface underneath are the typical picture of thrush. A white area that does not lift off when wiped and looks stuck to the tissue does not fit the typical course of thrush and needs to be examined.

Is thrush contagious?

Candida is already present in most healthy people's mouths, so it does not have to be passed on from outside for the condition to appear. What decides whether it appears is not transmission but the things that upset the balance in the mouth: antibiotics, steroid inhalers, a dry mouth, uncontrolled diabetes and wearing a denture.

Is the redness under my denture thrush, or is the denture rubbing?

It can be either, and the two are often found together. The distinction starts here: where a denture rubs, there is a painful sore confined to a single spot; with thrush, the redness covers the whole area the denture sits on, its border matches the border of the denture and it usually causes no pain.

Why does sleeping in a denture matter so much?

When you sleep in your denture, the tissue of the palate stays covered, moist and warm all day and all night; this environment suits the yeast. Studies of patients with denture-related oral thrush show that the great majority of them slept in their dentures. Taking the denture out at night is the simplest step for reducing recurrence, independently of any medicine.

I was treated for thrush but it came back. Why?

The commonest reason is that the denture was not treated. The yeast lives on the acrylic surface of the denture as much as in the tissue of the mouth; in one study of patients with denture stomatitis, half of the yeasts grown came from the surface of the denture. If the denture is not cleaned every day and taken out at night to soak in a suitable solution, the picture comes back in the same place once the medicine runs out.

How should I clean my denture?

The denture should be cleaned every day with a soft brush, on both the outer and the inner surface. At night it should be taken out and soaked in the solution your dentist or your pharmacist recommends. Boiling water can warp a denture; bleach and abrasive scouring powders damage it.

Does diabetes trigger oral thrush?

Yes. Blood sugar running high raises the sugar level in your saliva too, which makes it easier for the yeast to multiply. Recurring oral thrush is sometimes the first visible sign of uncontrolled diabetes; so with a recurring picture, review your blood sugar together with your doctor.

Does a dry mouth make thrush more likely?

Yes. Saliva acts as a natural cleanser in the mouth; when it decreases, an environment that suits the yeast forms. Blood pressure medicines, antidepressants and medicines used for bladder control can dry the mouth. Do not stop these medicines on your own; tell your doctor about the dryness.

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