Vaginal discharge that smells like spoiled milk is typically caused by an overgrowth of the beneficial bacteria that naturally live in your vagina. These bacteria, called lactobacilli, produce lactic acid to keep your vaginal environment slightly acidic. When they multiply too much, the excess lactic acid can create a distinctly sour, dairy-like smell. This is different from the “fishy” odor associated with more commonly discussed infections, which is why it can be confusing.
How Normal Vaginal Bacteria Create a Sour Smell
Your vagina naturally maintains a pH between 3.8 and 4.2, which is about as acidic as a tomato. Lactobacilli are the dominant bacteria responsible for this, and they do it by producing lactic acid and hydrogen peroxide. In normal amounts, this system keeps harmful bacteria and yeast from taking over, and healthy discharge has a mild, slightly tangy scent that most people barely notice.
But when lactobacilli overproduce, the balance tips too far in the acidic direction. The extra lactic acid is what gives the discharge that unmistakable spoiled milk or sour yogurt quality. Think of it like the same fermentation process that makes actual dairy go sour. Your vagina is essentially fermenting a little too aggressively.
Cytolytic Vaginosis: Too Much of a Good Thing
When lactobacilli overgrowth becomes significant enough to cause symptoms, it has a name: cytolytic vaginosis (CV). The term “cytolysis” refers to what happens when excess acid starts breaking down the walls of your vaginal cells. The overgrown bacteria literally attach to the surface of these cells and destroy them, releasing cell fragments into your discharge.
CV produces a vaginal pH between 3.5 and 4.5, which is at or below the normal range. The discharge itself can look white, frothy, or even cottage cheese-like, which is one reason it’s frequently mistaken for a yeast infection. Along with the sour smell, you may experience itching, burning, irritation, or discomfort during sex. These symptoms often worsen in the second half of your menstrual cycle, when progesterone levels rise and encourage lactobacilli to proliferate even more.
CV is not an infection. It’s an imbalance in the opposite direction from what most people expect. Instead of having too few good bacteria (as with bacterial vaginosis), you have too many.
How This Differs From Bacterial Vaginosis
The smell is actually the easiest way to tell these two conditions apart. Bacterial vaginosis (BV) produces a distinctly fishy odor, especially after sex. It happens when harmful bacteria outnumber lactobacilli, pushing the vaginal pH above 4.5. CV, by contrast, keeps the pH at or below 4.5 and produces that sour, milky scent instead.
This distinction matters because the treatments are essentially opposite. BV is treated with antibiotics that kill off harmful bacteria, giving lactobacilli room to recover. If you have CV and mistakenly treat it like BV, you’ll make the problem worse by wiping out competing bacteria and giving lactobacilli even more room to overgrow. The same problem occurs if CV is mistaken for a yeast infection. Antifungal treatments won’t help because there’s no fungus involved, and the underlying overgrowth continues unchecked.
Getting an Accurate Diagnosis
CV doesn’t show up on standard STI panels or routine lab work. It’s diagnosed through a wet mount, where a clinician examines a sample of your discharge under a microscope. What they’re looking for is specific: an unusually high number of lactobacilli, fragments of broken vaginal cells (called “bare nuclei” because the cell walls have been dissolved away), very few white blood cells, and no signs of yeast, trichomonas, or the bacteria-coated “clue cells” that indicate BV.
The CDC notes that a medical history alone isn’t enough to accurately diagnose the cause of vaginal symptoms, and many conditions that cause discharge, itching, and odor overlap in how they feel. If you’ve been treated repeatedly for yeast infections or BV without improvement, CV is worth raising with your provider. It’s underdiagnosed largely because it’s not as well known, not because it’s rare.
Managing the Overgrowth at Home
Because CV stems from too much acid, the goal of treatment is to gently raise your vaginal pH back into a comfortable range. The most commonly recommended approach is a baking soda sitz bath. Dissolve 2 to 4 tablespoons of baking soda in about 2 inches of warm water in a bathtub and soak for 15 to 20 minutes. During an active flare, you can do this twice daily. Once symptoms improve, tapering down to once or twice a week helps prevent recurrences.
A few habits can also help keep lactobacilli from overgrowing again. Avoid douching or using acidic vaginal washes, since these lower pH further. If you use probiotics that contain lactobacillus strains, consider pausing them temporarily to see if your symptoms improve. Wearing breathable cotton underwear and avoiding prolonged moisture (from workout clothes or wet swimsuits) reduces the warm, enclosed environment where bacteria thrive.
Other Possible Causes of a Sour Smell
Not every spoiled milk scent points to CV. A few other situations can produce a similar odor:
- Normal hormonal shifts. Discharge naturally changes in consistency and scent throughout your cycle. A mildly sour smell around ovulation or just before your period can be completely normal, especially if you have no itching, burning, or unusual color.
- Sweat and skin bacteria. The groin area has a high concentration of sweat glands. Sweat mixing with normal skin bacteria can produce a sour, dairy-like smell that seems to come from your discharge but is actually external.
- Diet. Foods rich in sulfur or strong spices can temporarily alter body odor, including vaginal scent. This is usually short-lived and resolves on its own.
The key factors that distinguish a simple fluctuation from something worth investigating are persistence and accompanying symptoms. A sour smell that lasts more than a week, comes with itching or burning, or accompanies discharge that looks different from your baseline is worth a closer look. A temporary change that resolves on its own, especially if it lines up with your cycle, is generally your body doing what it does.