Yeast infections are common conditions that can pop up in many areas of the human body, including the vulvovaginal region. They are usually caused by a fungus called Candida albicans, which starts to grow profusely, leading to the white discharge associated with yeast infections. Fungi are not killed by antibiotics, which are only effective against bacteria. As such, yeast infections may be encouraged when their bacterial competitors are wiped out by antibiotics — especially broad-spectrum antibiotics. Candida albicans can also grow on other areas of the body; for instance, when it proliferates in the mouth, the resulting condition is called thrush.

The Lactobacillus species in yogurt are different from those found in the vagina.

The vagina is habitat to bacteria from the Lactobacillus genus, members of which produce lactic acid and sometimes hydrogen peroxide. This helps to inhibit the growth of bacteria that aren’t able to thrive in acidic environments or in the presence of hydrogen peroxide. If you have a vagina, there is about a 10 to 25 percent chance that yours is home to Candida albicans — but this doesn’t necessarily mean you’ll develop a yeast infection. The lactobacilli are usually able to keep C. albicans in check.

Yogurt is often touted as a cure or preventive measure for yeast infections. Yogurt is milk that has been inoculated with bacteria that have been allowed to grow. When the yogurt is being manufactured, it is held at a temperature that allows the bacteria to thrive; when yogurt is kept in the refrigerator, the bacteria don’t die, but they aren’t able to reproduce either. Don’t worry, these bacteria won’t harm you — such bacteria, when used in foods or supplements, are often referred to as “probiotics.”

Two important species that make up yogurt are Streptococcus thermophilus and Lactobacillus delbrueckii bulgaricus, both of which produce lactic acid. S. thermophilus eats sugar and produces acid as a byproduct. So does L. d. bulgaricus, which also plays more of a role in the yogurt’s flavor and aroma. Eventually, the lactic acid produced by the bacteria decreases the pH of the yogurt so much that other microorganisms are unable to survive. This method of fermentation has been used for millennia as a way to ensure that milk is safe to eat.

Yogurt is thought to be beneficial because it contains lactobacilli as part of its live cultures — some believe that lactobacilli ingested in yogurt are able to survive the trip through the digestive system and then migrate from the anus to the vagina, where they can populate the urogenital tract. Some people use plain, unsweetened yogurt as a topical remedy, applied directly into the vagina; for this to work, the lactobacilli in the yogurt would need to be able to adhere to the cells in the vagina. However, while L. acidophilus — a common yogurt ingredient — can survive the journey through the acid- and enzyme-drenched digestive system, it does not have a very strong ability to adhere to (and thereby colonize) the vaginal cells.

The most common Lactobacillus species in the vagina are Lactobacillus crispatus, L. jensenii, L. gasseri, and L. iners, which are different species from those found in yogurt. For instance, the Amande-brand yogurt that is sitting before me at the moment includes four lactobacilli species: L. acidophilus, L. bulgaricus, L. casei, and L. rhamnosus.

According to a 2006 review of the literature, so far there is not very much evidence that the types of “probiotics” found in yogurt are effective against yeast infections. Most studies that have been done were methodologically flawed — they often had very few test subjects, and often these subjects would drop out of the studies before their completion; some studies were not double-blinded or placebo-controlled. This means that large-scale, quality studies will need to be done to give us the answers we want. Of the studies reviewed, results were conflicting, with some experiments seeming to demonstrate a benefit from yogurt while others showed no benefit. It seems that, so far, we just can’t draw any solid conclusions.

Some laboratory experiments have shown that certain lactobacilli species can inhibit C. albicans’s growth or ability to adhere to vaginal cells — but simplified lab techniques don’t necessarily reflect what is happening in the human body. In any case, the results of some of these studies were interesting. One found that L. delbrueckii (also known as L. bulgaricus) was very effective in inhibiting the growth of C. albicans. Another study showed that L. acidophilus seemed to decrease C. albican’s ability to adhere to vaginal epithelial cells. However, a test tube or Petri dish is not necessarily an adequate model of the urogenital tract. These laboratory experiments are intriguing — but due to the methodologically flawed nature of the human studies, we can’t jump to any conclusions about yogurt’s efficacy in treating or preventing yeast infections. There is just not enough evidence.

There are other simple preventive measures you can take to avoid yeast infections, and over-the-counter medications are available to cure any yeast infections you are unfortunate enough to develop.

Is yogurt useless? Of course not. Many people enjoy the taste of yogurt, and there are varieties based on dairy milk, soy milk, coconut milk, and almond milk, making yogurt appealing to people with a range of dietary requirements. Large-scale, well-designed studies on the use of probiotics to treat or prevent yeast infections might be carried out in the future, and it will be interesting to learn about their results. The worst-case scenario is that yogurt is just a delicious and healthful treat.

Editor’s Note: For another evidence-based discussion of popular home remedies, see our post on cranberry juice and urinary tract infections (UTIs).