Causes and Risk Factors of Thrush

Oral thrush is typically caused by a yeast called Candida albicans. It normally is found in the mouth but its population is kept in balance by your immune system and the other bacteria found there. The yeast can grow out of control and cause oral thrush when your immune system is weakened or your oral bacteria are killed by antibiotics.

Oral thrush is sometimes seen in newborns because their immune systems have not yet fully formed. They may have diaper rash due to Candida at the same time.

Common Causes

Though Candida albicans is the most common yeast involved in thrush, it may also be caused by similar types, such as Candida glabrata or Candida tropicalis. Your risk of oral thrush is increased by certain conditions, medications, and treatments that weaken the immune system or upset the normal balance of saliva, bacteria, and yeast in your mouth.

Medications and Treatments

• Oral corticosteroids: When taken long-term for a variety of conditions, these drugs weaken the immune system.
• Inhaled steroids: Inhaled steroids are used for asthma and chronic obstructive pulmonary disease (COPD). They can raise your risk for thrush.
• Chemotherapy and radiotherapy: Your immune system is weakened while you are under chemotherapy or you have radiotherapy to the head and neck.
• Immunosuppressive treatment in organ transplantation: Patients are treated to prevent rejection after an organ transplant and this can increase the risk of thrush.
• Antibiotics: Antibiotics typically kill the oral bacteria but they don't act against yeast, which is a type of fungus. A newborn is more at risk if the baby or the mother (if breastfeeding) takes antibiotics.
• Medications that reduce saliva: Your saliva carries antibodies and other substances that help prevent overgrowth of the yeast, so medications that reduce saliva also increase the risk of thrush.

Health Concerns

• HIV or AIDS: Thrush can be seen in HIV infection even while someone is under antiretroviral therapy. Candida can grow out of control and become invasive when the CD4 count is less than 200 cells/mL in people with AIDS.
• Immune disorders: Other immune disorders also increase the risk of thrush, including blood cancers such as leukemia and lymphoma.
• Diabetes: High blood sugar with diabetes increases the risk of thrush.
• Long-term illness: Having a chronic condition can weaken the immune system and increase your risk of thrush.
• Oral conditions: Dentures that do not fit properly and damage the mucous membranes can increase risk. Having a condition that causes dry mouth, such as Sjogren's syndrome, is also a risk factor.
• Pregnancy and newborns: The mother's immune system is reduced during pregnancy and the baby's immune system takes months to be fully up to speed. A newborn may pick up Candida during birth if the mother has a vaginal yeast infection, or acquire it after birth. Often thrush is only a minor irritation for a baby. However, frequent oral thrush should always be investigated in infants to find a cause.

Lifestyle Risk Factors

Poor oral hygiene increases your risk of thrush. Brush your teeth twice a day and clean between your teeth daily. If you have dentures, ensure you are cleaning them daily and brushing your tongue and gums.

Smoking tobacco upsets your oral health and increases your risk of oral thrush, although it isn't clear exactly why. This is one more health reason to stop smoking. There are anecdotal reports (but no clinical studies) that smoking cannabis also raises the risk of thrush.

If you have type 1, type 2, or gestational diabetes, it is important to maintain good blood sugar control through medication and diet.

If you use a steroid inhaler, you may reduce your risk of thrush by rinsing your mouth and brushing your teeth after use of the inhaler. If you use a metered-dose inhaler for steroids, a spacer—a chamber placed between your inhaler and your mouth that allows you to more effectively breathe in the medicine—may help prevent or alleviate the symptoms of thrush. 

The spacer helps channel the treatment into the lungs and reduces exposure in the mouth. However, dry powder inhalers (such as Advair, Pulmicort, and Asmanex) don't use a spacer, and the powder isn't as easily cleared by brushing after use. In this case, you may want to use an alcohol-based mouthwash like Listerine to help rinse out your mouth.