Cloudy urine is most often caused by a harmless buildup of phosphate crystals that form when your urine becomes alkaline, typically after eating a big meal or drinking a lot of milk. But it can also signal an infection, dehydration, or other conditions worth paying attention to. The cloudiness itself comes from particles suspended in the urine: crystals, proteins, white blood cells, bacteria, or other cellular material that scatter light instead of letting it pass through clearly.
Phosphate Crystals: The Most Common Cause
The single most common reason for cloudy urine is phosphate precipitation in alkaline urine. When the pH of your urine shifts toward the alkaline side, dissolved phosphate minerals come out of solution and form tiny crystals. These crystals make your urine look milky or hazy, even though nothing is wrong.
This happens most often after meals, especially if you’ve consumed a lot of dairy. It can also occur if you take vitamin supplements that contain phosphate. The cloudiness is intermittent, meaning it comes and goes depending on what you’ve eaten and how your body is processing it. If your urine looks cloudy once in a while but clears up on its own and you feel fine otherwise, phosphate crystals are the likely explanation.
Urine that sits out at room temperature or in the refrigerator can also turn cloudy as salts crystallize and precipitate. So if you noticed cloudiness in a sample that wasn’t fresh, that alone could explain it.
Dehydration and Concentrated Urine
When you’re not drinking enough water, your kidneys conserve fluid by producing less urine. That smaller volume of urine carries the same amount of waste, making it more concentrated. Concentrated urine looks darker and can appear cloudy because there’s a higher density of dissolved substances that may begin to precipitate. Drinking more fluids typically clears this up within a few hours. Pregnant women are especially prone to this because their fluid needs increase significantly.
Urinary Tract Infections
A UTI is one of the most well-known causes of cloudy urine, and it’s the one most people worry about when they notice the change. The cloudiness in this case comes from white blood cells flooding into the urinary tract to fight bacteria, along with the bacteria themselves. When white blood cell counts in the urine are high enough to be visible, the condition is called pyuria, and it’s one of the hallmark signs of infection.
A UTI rarely causes cloudiness alone. You’ll usually also notice a burning sensation when you urinate, a frequent urgent need to go, or pelvic pressure. If bacteria are present in significant numbers (generally over 100,000 organisms per milliliter), a lab test will also show positive results for nitrites, which bacteria produce as a byproduct. If your cloudy urine comes with pain, burning, or fever, an infection is a strong possibility.
Sexually Transmitted Infections
STIs like gonorrhea, syphilis, and HPV can also cause pyuria, sometimes in a form called “sterile pyuria.” This means your urine is full of white blood cells, making it cloudy, but a standard urine culture won’t grow typical UTI bacteria. If you’re being treated for what seems like a UTI but the antibiotics aren’t working and the cloudiness persists, an STI may be the underlying cause. Discharge from the infection can also mix with urine as it leaves the body, adding to the hazy appearance.
Blood Sugar and Diabetes
Diabetes can cause sugar to build up in your urine. When blood sugar levels are high enough that the kidneys can’t reabsorb all the glucose, the excess spills into the urine. High concentrations of sugar can make urine appear cloudy. If you’re noticing persistently cloudy urine alongside increased thirst, frequent urination, or unexplained fatigue, elevated blood sugar is worth investigating.
Vaginal Discharge and Pregnancy
During pregnancy, the body produces more vaginal discharge (called leukorrhea). This discharge can mix with urine in the toilet or in a collection cup, making the urine look cloudy even when the urine itself is perfectly clear. Hormonal changes and increased pressure from the growing uterus on the bladder also make pregnant women more susceptible to actual UTIs, which adds another layer of concern. If you’re pregnant and noticing cloudy urine, it’s worth distinguishing between discharge mixing in and a true change in the urine itself. Collecting a “clean catch” midstream sample can help clarify the difference.
Outside of pregnancy, normal vaginal discharge can cause the same visual effect, particularly if you notice cloudiness only in the toilet bowl rather than in a clean sample.
Retrograde Ejaculation in Men
Men who notice cloudy urine specifically after orgasm may be experiencing retrograde ejaculation. This happens when semen travels backward into the bladder instead of exiting through the penis. The semen then mixes with urine, making the next urination look milky or cloudy. It’s not dangerous, but it can affect fertility, and it’s sometimes a side effect of certain medications or surgeries.
Medications and Supplements
Certain medications and supplements can change the appearance of your urine. Phosphate-containing vitamin supplements are a known cause, essentially mimicking the same crystal formation that happens after a dairy-heavy meal. If the cloudiness started around the same time you began a new supplement or medication, that timing is a strong clue.
What the Cloudiness Looks Like Matters
Labs classify urine clarity on a simple scale: clear, mildly cloudy, cloudy, or turbid. Mildly cloudy urine that happens once after a large meal and resolves on its own is almost always harmless phosphate precipitation. Persistently cloudy or truly turbid urine, especially when paired with pain, odor, fever, or blood, points toward something that needs evaluation. Cloudy urine with flank pain (pain in your side or lower back) and fever can indicate a kidney infection, which is more serious than a simple bladder infection and needs prompt treatment.
A simple urinalysis can sort out the cause quickly. It checks for white blood cells, bacteria, protein, sugar, and crystals, giving a clear picture of what’s making your urine hazy. If the cloudiness has been happening repeatedly or you have other symptoms alongside it, that test is the fastest route to an answer.