Reviewed October 2020

What causes thrush? We asked a doctor


Thrush is a common condition and one most women will have experienced in their time. For some, it can be a passing irritation, while for others it can occur often and occasionally enough to have quite an impact on quality of life. Regardless of whether or not you have had it, you will certainly have heard it talked about. But what actually is it and why does it occur?

There are a certain set of microbes, including bacteria, viruses, and fungi, that inhabit the skin and mucous membranes of human bodies. Known as the normal flora of the body, they often live in harmony with humans, causing no disease. Some are even beneficial to humans.

However, some of them are opportunistic, and can cause disease when conditions permit them to. Candida, which causes thrush, is one such fungus which acts as an opportunistic pathogen in the human body, mainly residing on the skin and inner lining of the gut, genital, and urinary tracts.

When certain factors in the immediate environment of the Candida fungus change, favouring their growth or when the human host’s immunity drops, this can lead to an overgrowth of Candida, which causes an infection called Candidiasis, otherwise known as thrush. Out of the many types of Candida species, Candida albicans is the most likely to cause Candidiasis.1

Symptoms of thrush

Symptoms and signs of thrush depend on the area of the body where the Candida fungus has multiplied. Overgrowth of Candida in the mouth and throat is called oropharyngeal candidiasis, or oral thrush. It can lead to a red and sore mouth, and the corners of the mouth may get cracked. You may also notice white patches on the tongue, oral cavity, or even throat, and swallowing may become painful.

Infection in the vagina or genital region is called genital or vulvovaginal candidiasis. Symptoms include a thick curd-like vaginal discharge, vaginal itching, and pain during urination or sexual intercourse.

Less commonly, Candida can enter the bloodstream and spread throughout the body, causing invasive candidiasis. Usually immunocompromised patients who are already in hospital are at risk as Candida can enter the bloodstream through medical equipments and invasive procedures. Symptoms are nonspecific and include fever with chills. It can cause problems in any part of the body, including the heart and brain, and can therefore become a serious condition.

What makes you more susceptible to thrush?

There is no single specific cause for thrush, but there are certain factors which favour infection. Candidiasis is common in those with a weak immune system, as when your immune system does not function properly, the normal defence mechanisms which fight against Candida growth are disrupted. Elderly people and newborns have poor immunity, and certain illnesses like HIV, cancer, and uncontrolled diabetes can weaken the immune system. Medications like corticosteroids and drugs used to treat cancer are known to suppress human immunity.

Candida growth is also kept under control by other bacterial microbes in the normal flora, for example Lactobacillus in the vagina, via mechanisms such as competing for nutrients and secreting antifungal substances.2,3 When you take long-term antibiotics to treat a bacterial infection, especially broad spectrum antibiotics, they not only kill the harmful bacteria causing disease but also the good ones in the normal flora. This disruption of the natural balance of microbes can lead to unrestrained overgrowth of Candida.

Candida growth is kept under control by other bacterial microbes in the normal flora, for example Lactobacillus in the vagina, via mechanisms such as competing for nutrients, and secreting antifungal substances

Other local factors in the oral or vaginal environment can favour Candida growth as well. Oestrogen is known to promote the growth of Candida through various mechanisms, including stimulating the production of glycogen within vaginal cells which is a perfect food source for Candida, and acting on Candida to help them better adhere or cling onto the vaginal wall.4 This is the reason behind increased vaginal thrush in women in hyper-oestrogenic states such as pregnancy, and those using combined oral contraceptive pills or hormone replacement therapy.

Vaginal pH, or in simple terms, the level of acidity or alkalinity of the vagina, has a role in Candida growth. The normal acidic pH in the vagina inhibits unnecessary proliferation of Candida, but when this acidity is reduced, altering the pH, growth of Candida is likely to increase.5 Presence of alkaline substances like menstrual blood or semen within the vagina are known to increase the vaginal pH level.

Which lifestyle factors can affect your risk of getting thrush?

There may be other simple things which you do on a daily basis which, unbeknown to you, may be promoting Candida growth in your body, and changing some of these lifestyle factors may help prevent infection if you find yourself getting thrush often. These lifestyle acts may not necessarily cause Candida overgrowth in your vagina themselves, but they may add to the risk if you are already prone to infection.

Wearing tight clothes and underwear made out of synthetic material is best avoided if you have thrush or get it often. These are known to trap heat and moisture within your vagina, and as with any other fungus, the warm, moist environment is perfect for Candida to grow. The same applies with wet swimsuits and gym clothes, which you should change out of as soon as possible after exercise. If you find you get thrush often, you’re better off wearing cotton underwear and loose clothes to allow for more ventilation.

You may have heard that vaginal douching and using vaginal cleansers can help keep your vagina clean, but these actually do more harm than good. They disrupt the natural balance of the normal vaginal flora and alter the vaginal pH, which in turn may promote the growth of Candida in the vagina. For this reason, these practices are discouraged and frequent change of underwear and cleaning only the vulval area as opposed to inside the vagina is sufficient for good vaginal hygiene.

If you notice that your thrush tends to flare up with menstruation, it may be due to menstrual blood altering the vaginal pH

If you notice that your thrush tends to flare up with menstruation, it may be due to menstrual blood altering the vaginal pH. This can be aggravated by the use of tampons as they trap blood within the vagina for hours at a time. Changing tampons frequently or even avoiding their use is advisable in such cases.

Although thrush is not a sexually transmitted disease, some women complain that their thrush worsens after sexual intercourse. Semen or some lubricants, which alter the vaginal pH, could be the culprit here, so avoiding such lubricants and using condoms could help. You can discuss this with your GP and ask them to prescribe you a safer pH-balanced lubricant, free of ingredients like glycerin and parabens.

Oral thrush and oral hygiene

While the generalised factors mentioned above, such as poor immunity and antibiotic use, play a major role in development of oral thrush as well as vaginal thrush, there are some local factors related to oral health and hygiene which may contribute. Smoking, having a dry mouth, and dentures can cause irritation in your mouth, and damage to the oral mucosa may make it easier for Candida to invade. Research also claims that there is a correlation between oral thrush and poor oral hygiene,6 hence practises like brushing teeth and keeping your dentures clean are important.

When should I see a doctor?

You may be able to self-diagnose yourself with thrush or narrow down the triggering factors which may be affecting you. However, it is important to note that although thrush is common, what you think is thrush could be something else, for example an irritation of the genital area, or a completely different bacterial infection.

It’s always better to visit your GP if you are concerned and get proper treatment with antifungals, especially if it is the first time you have got thrush or you find you are getting it very often, as this may require further investigation. This will also give you the opportunity to discuss your triggers and risk factors and get advice on the best alternatives for your situation.

Featured image is of a woman resting her chin on her fists, as if thinking. She is cropped so that she is visible from the chin down to her elbows. The background is a left-to-right gradient that moves from blue to white

Last updated October 2020
Next update due 2022

Dr. Shehara Ariyaratne, MBBS

Shehara graduated from the University of Colombo, with degrees in Medicine and Surgery. She has research experience in various fields of medicine, having worked at the Epidemiology Unit of Sri Lanka and National Institute of Infectious Diseases. She is currently practising paediatrics and radiology at the Lady Ridgeway Hospital, Colombo, the main paediatric hospital in the country. Though a medical doctor in profession, writing has always been her passion and she hopes her articles will help empower women.

View more


  1. NICE, Candida – female genital, Clinical Knowledge Summary, National Institute for Health and Care Excellence, May 2017 [online] (accessed 23 October 2020) 
  2. Jae Jang, S., et al., Vaginal lactobacilli inhibit growth and hyphae formation of Candida albicans, Scientific Reports, 2019, no 9, p 8121 [online] (accessed 23 October 2020)
  3. Zangl, I., et al., The role of Lactobacillus species in the control of Candida via biotrophic interactions, Microbial Cell, January 2020, vol 7, issue 1, pp 1-14
  4. Peters, B.M., et al., Candida Vaginitis: when opportunism knocks, the host responds, PLOS Pathogens, 2014, Vol. 10, No. 4, e1003965
  5. Ibid.
  6. Muzurovic, S., et al., The relationship between oral hygiene and oral colonisation with Candida species, Medical Archives, 2012, Vol. 66, No. 6, pp. 415-417

Creating genuinely useful health information is important to us and we value your feedback!
Was this article helpful, educational, or easy to understand? Email: