Thrush develops when a yeast called Candida albicans, which naturally lives on your skin and inside your body, grows out of control. You don’t “catch” it in most cases. The fungus is already there, kept in check by the balance of healthy bacteria in your body. When something disrupts that balance, the yeast multiplies and causes an infection.
The Balance That Keeps Thrush in Check
Candida albicans lives in your mouth, gut, and genital area as part of your normal microbiome. Healthy bacteria act as a natural control system, competing with the yeast for space and resources. As long as these bacteria are thriving, Candida stays at low, harmless levels.
When something knocks this balance off, Candida takes advantage. The yeast shifts from a passive resident to an active infection, multiplying rapidly in warm, moist areas like the mouth, throat, or genitals. This is why thrush isn’t really about exposure to the fungus. It’s about what changed inside your body to let it overgrow.
Antibiotics Are a Major Trigger
Antibiotics kill bacteria, and they don’t discriminate well between harmful bacteria and the protective bacteria that keep Candida in check. A study tracking 650 women over 18 months found that penicillin use made yeast infections roughly four times more likely. Other common antibiotics raised the risk too, with certain types nearly tripling it. The broader the antibiotic and the longer the course, the more your protective bacteria are depleted.
This is why thrush commonly shows up during or shortly after a round of antibiotics, even if you’ve never had it before. The yeast was always present. The antibiotics just removed the competition.
How Diabetes and High Blood Sugar Fuel Yeast
Candida feeds on sugar. If your blood sugar runs high, the extra glucose shows up in your saliva, sweat, and urine, creating an ideal environment for yeast to thrive. This is why people with diabetes, particularly those whose blood sugar isn’t well controlled, get thrush more frequently than the general population.
Recurrent thrush can even be an early warning sign of undiagnosed diabetes. If you’re getting yeast infections repeatedly without an obvious cause like antibiotic use, it’s worth checking your blood sugar levels.
Inhaled Steroids and Oral Thrush
If you use a steroid inhaler for asthma or COPD, you have a notably higher chance of developing oral thrush. Research on COPD patients found that those using inhaled corticosteroids had roughly double the odds of developing oral thrush compared to those on non-steroid inhalers. The risk also increases with higher doses.
The reason is straightforward: the steroid residue that lands in your mouth and throat suppresses the local immune response, giving Candida room to grow. Rinsing your mouth with water and spitting after every puff significantly reduces this risk. The type of inhaler device also matters. Metered-dose inhalers (the pressurized canister type) were associated with about a third fewer thrush cases than dry powder inhalers in one large study, likely because of how the medication is deposited in the mouth.
Other Common Risk Factors
- Weakened immune system. Conditions like HIV, cancer treatment, and organ transplant medications all suppress the immune system’s ability to keep Candida under control.
- Dentures. Poorly fitting dentures create warm, moist pockets where yeast thrives, especially if dentures aren’t cleaned thoroughly each day.
- Pregnancy and hormonal changes. Shifts in estrogen levels change the vaginal environment in ways that favor yeast growth, which is why vaginal thrush is more common during pregnancy.
- Dry mouth. Saliva contains antimicrobial compounds that help control Candida. Medications that cause dry mouth, or conditions that reduce saliva production, raise the risk of oral thrush.
- Smoking. Tobacco use alters the balance of organisms in the mouth and damages the lining of the mouth, making oral thrush more likely.
Can You Get Thrush From Another Person?
Thrush is not classified as a sexually transmitted infection. That said, sex can trigger it, and in some cases, it can be passed between partners during sex. If your partner develops symptoms, they should get treated, but you don’t need to treat a partner who has no symptoms.
Thrush and Breastfeeding
Breastfeeding is the one situation where thrush does pass back and forth between two people fairly easily. A baby with oral thrush can transfer the yeast to the mother’s nipples during feeding, and the mother can pass it right back at the next feed, creating a cycle that’s hard to break without treating both at the same time.
In babies, the signs include creamy white patches on the tongue, gums, or inside the cheeks that don’t wipe off with a clean cloth. You might also notice a white film on the lips, fussiness during feeds, or a persistent diaper rash that won’t clear up. In mothers, thrush typically causes a burning pain in the nipples (especially after feeding), itchiness, flaking or shiny skin around the nipple, and sometimes white patches or tiny blisters on the areola.
Why Some People Get Thrush Repeatedly
A single episode of thrush usually traces back to one clear trigger: a course of antibiotics, a new inhaler, a stretch of poorly controlled blood sugar. Recurrent thrush, typically defined as four or more episodes in a year, points to something more persistent. That could be an underlying condition like diabetes, ongoing medication use, or a chronically weakened immune system.
Some people are also simply more prone to Candida overgrowth because of the composition of their microbiome. If thrush keeps coming back despite addressing obvious triggers, it’s worth investigating deeper causes rather than just treating each episode as it appears.