Causes of Thrush: Oral, Vaginal, and Balanitis

Summary

Thrush is caused by infection with a type of yeast called Candida - it's sometimes called a yeast infection. It's most commonly due to the species Candida albicans, although other Candida species can cause thrush too. Candida normally lives harmlessly in our bodies. It's normal to find it in the mouth, on the skin, in the vagina, and on the penis, amongst other places, without it causing any problems.

However, sometimes it can cause infection. This happens if there is a problem with the immune system, a problem with the skin, or a disruption in the balance of healthy bacteria, allowing Candida to grow and invade the skin or lining of the mouth/vagina/penis, causing infection.

Causes of oral thrush include:

Having a weakened or under-developed immune system due to age - for instance in babies and in older people.
Taking antibiotics.
Wearing dentures, especially if they don't fit properly, or aren't cleaned properly.
Poor oral hygiene - for example, not brushing teeth regularly.
Having a dry mouth.
Using inhaled steroids or steroid tablets.
Smoking.
Having under-controlled diabetes.
Having an underactive thyroid.
Having a weakened immune system - for example taking medications that suppress the immune system, or having a condition that affects the immune system, such as HIV.

Causes of vaginal thrush include:

Taking antibiotics.
Having under-controlled diabetes.
Taking certain medications, such as steroid tablets and medicines such as dapagliflozin.
Having high levels of oestrogen - for example, using combined hormonal contraception, being pregnant, or using hormone replacement therapy.
Having a weakened immune system - for example, taking medications that suppress the immune system, or having a condition that affects the immune system, such as HIV.
Stress - possibly.
Using things that irritate and damage the skin of the vagina and vulva, such as harsh soaps, shampoos, and douching.

Causes of thrush affecting the penis (Candidal balanitis) include:

Poor hygiene.
Having a foreskin that doesn't pull back fully.
Taking antibiotics.
Having under-controlled diabetes.
Taking certain medications, such as steroid tablets and medicines like dapagliflozin.
Having a weakened immune system - for example, taking medications that suppress the immune system, or having a condition that affects the immune system, such as HIV.
Using things that irritate and damage the skin on the head of the penis, such as harsh soaps and shampoos.

Causes of thrush affecting the skin - usually causing infection of the skin folds, called intertrigo - include:

Obesity and overweight.
Excessive sweating.
Wearing tight-fitting clothing.
Hot and humid environments.
Having a weakened immune system.
Nappies - babies can get Candida infection sometimes, which looks similar to nappy rash.

Candida infections that affect deeper parts of the body, and cause more serious infections, occur in people with seriously weakened immune systems, such as those on certain types of chemotherapy, or with AIDS.

In this series of articles centred around thrush you can read about thrush causes, thrush treatment, and symptoms of thrush - all written by one of our expert GPs.

The rest of this feature will take an in-depth look at the causes of thrush, as, at Patient, we know our readers sometimes want to have a deep dive into certain topics.

Thrush symptoms

Thrush is a common infection. It's caused by an infection with a type of yeast - which are also ...

What causes thrush?

Causes of oral thrush

It's normal for Candida to live in the mouth. Most people never get any symptoms of infection. Reasons for oral thrush to develop include:

Being very young (newborn babies) or older (older adults). Newborn babies' developing immune system often can't control thrush. It's thought that in older people, the immune system weakens and is less able to fight off thrush infection. Older people are also more likely to have a dry mouth, which can make the mouth more vulnerable to thrush.
Taking antibiotics, particularly broad spectrum antibiotics. These are antibiotics which kill a lot of different types of bacteria. Killing the good bacteria that live in the mouth can let Candida grow out of control, and lead to thrush.
Using steroid medicines - such as steroid tablets or steroid inhalers. These can weaken the immune system in the mouth, allowing Candida to grow and cause thrush. Rinsing or gargling with water after using an steroid inhaler can help to stop this happening, as well as using a spacer. A spacer helps the steroid to get into the lungs, and not stick in the mouth.
Having poor dental hygiene. Regularly brushing teeth twice a day, flossing, and seeing a dentist for checkups can help to prevent oral thrush.
Wearing dentures. This is a particular problem if they don't fit properly. It's thought that Candida can attach to denture material, and that a dry mouth with dentures can make it more likely to develop thrush.
Having a severely weakened immune system, for example as a result of:

Having blood cancer - such as leukaemia.
Having chemotherapy and/or radiotherapy as cancer treatment.
Having HIV/AIDS.

Having uncontrolled diabetes (type 1 or type 2).
Smoking is an important cause of thrush. It's not exactly clear why, but one theory is that smoke makes the lining of the mouth more vulnerable to Candida growth and infection.
Having a dry mouth for any reason. Saliva contains anti-microbial proteins that stop Candida from growing too much.
Having a poor diet. Eating too much refined sugars, carbohydrates and lactose-containing dairy products might affect the environment inside the mouth, causing Candida to grow more easily.
Being deficient in iron or other vitamins, such as vitamin B12.

Causes of vaginal thrush

It's also normal to have Candida species living in the vagina without causing any symptoms. Vaginal and vulval thrush is very common - around three out of four women get it at least once in their lives - and the exact cause is often not clear.

Sometimes, a cause can be identified. These include:

Taking antibiotics, particularly broad spectrum antibiotics that can disrupt the balance of good bacteria in the vagina.
Having under-controlled diabetes. Uncontrolled diabetes can affect healing. Having sugar in the urine - which occurs if your blood sugars are very high - can cause Candida to grow.
Using steroid tablets, which reduce the activity of the immune system.
Taking medicines such as dapagliflozin or empagliflozin (SGLT2 inhibitors), for diabetes or heart failure. These work by causing glucose (sugar) to enter the urine - which Candida can feed off to grow.
High oestrogen levels. Vulval and vaginal thrush is probably linked to oestrogen - it's rare in pre-pubertal girls and uncommon after the menopause, when oestrogen levels are normally low. Things that cause higher oestrogen levels, such as combined hormonal contraception, HRT (hormone replacement therapy), and pregnancy, can be linked with thrush.
Having a severely weakened immune system. As with oral thrush, this might be due to HIV/AIDS, or many other conditions.
Using things that irritate the vulva and vagina. Harsh soaps, shampoos, shower gels, and vaginal douching can all irritate the lining and skin of the vagina and vulva, damaging it and making it easier for Candida to cause infection.
Stress is possibly linked to thrush. Stress can increase levels of cortisol, a hormone which stops the immune system working as well, so this could lead to thrush. There isn't much strong evidence of this link occurring in practice, though.

Causes of thrush affecting the penis

Thrush affecting the penis is less common than vulvovaginal thrush. Causes of thrush affecting the penis include:

Poor hygiene. For example, not cleaning the penis and underneath the foreskin.
Having a foreskin that doesn't fully pull back. This can make it difficult to clean underneath it. Young boys often have a foreskin that doesn't fully pull back - usually it does as they get older. Men who are circumcised are less likely to get thrush affecting the penis.
Taking antibiotics that disrupt the balance of good bacteria on the penis.
Having under-controlled diabetes - this can lead to glucose - sugar - in the urine, which Candida uses to grow.
Having an impaired immune system due to another condition, like HIV/AIDS or chemotherapy.
Taking steroid tablets, which reduce the activity of the immune system.
Taking dapagliflozin, empagliflozin or other SGLT2 inhibitors, which cause glucose (sugar) to pass out into urine.
Sometimes, men can develop thrush after sex. Candida isn't considered a sexually transmitted infection - although it's possible to pass it on through sex, Candida is also present over most of the body anyway, and most people carry it or pick it up from the environment in other ways. Sex might cause irritation to the head of the penis, perhaps causing symptoms of thrush, or setting off an infection.

Thrush in other areas

Candida can also cause an infection on the skin. One example is intertrigo - infection between skin folds. Causes of this include:

Obesity and overweight. This can lead to larger skin folds which might become damp and warm - perfect conditions for Candida to grow.
Excessive sweating. Likewise, this makes the skin damp and therefore it becomes easier for Candida to grow.
Wearing tight-fitting clothing.
Being in a hot and humid environment.
Having a weakened immune system for another reason.
Nappies can also be warm, damp environments and babies can get Candida infection, which looks similar to nappy rash.

Candida can cause deeper infections, such as in the oesophagus (food pipe/gullet), or in the bloodstream. These mostly occur in people who are seriously ill for another reason, or people with severely weakened immune systems, such as people with HIV/AIDS or people on chemotherapy.

Can antibiotics cause thrush?

Antibiotics are a common cause of thrush. All of us have bacteria and other organisms living on, and in, our bodies. These normally exist in a healthy balance, preventing bad organisms from growing too fast and causing infection. Broad-spectrum antibiotics, that kill a large range of organisms, are especially likely to disrupt this balance by killing good bacteria. If this happens in the vagina, for example, Candida can grow out of control without good bacteria keeping it at bay, leading to symptoms of thrush.

Broad-spectrum antibiotics include amoxicillin, co-amoxiclav, and ciprofloxacin.

How common is thrush?

Thrush is common with three out of every four women getting at least one episode of vaginal thrush during their lives, and around two in four women having two or more episodes of thrush.

Oral thrush affects around 15% of newborns and around 10% of older people, but is less common in other age groups.

Thrush affecting the penis is fairly common, although not as common as vaginal thrush. It's difficult to put a figure on how many men have it, but around one in every 25 boys get balanitis - inflammation of the head of the penis - at some point, of which thrush is a common cause.

Candida infections of the skin - such as intertrigo - are fairly common as well, although again it's difficult to say exactly how common they are. It can be more common in those that are bed-bound - especially in areas that are harder to keep dry such under the breasts or in the groin creases.

Invasive or deeper Candida infections are rare, and typically only found in people with severely weakened immune systems.

Thrush risk factors

Risk factors for thrush - mouth, genital, and skin - are all related to the causes of thrush, as listed above. They include:

Age.

Oral thrush is more common in babies and older adults.
Vaginal yeast infections are more common in women between puberty and the menopause - they're rare before puberty and less common after menopause.

Diabetes.
Using antibiotics.
Using steroid medicines.
Having a severely weakened immune system for another reason.

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