Riboflavin, also known as vitamin B2, is the vitamin responsible for turning your pee bright yellow. It’s a water-soluble vitamin, which means your body doesn’t store large amounts of it. When you take in more than your body needs, your kidneys flush the excess out through urine, and riboflavin’s natural fluorescent yellow pigment comes along for the ride.
Why Riboflavin Turns Urine Neon Yellow
Riboflavin is one of the few vitamins with a strong natural color. The word “flavin” actually comes from the Latin word for yellow. When your body absorbs what it needs from riboflavin, the surplus passes through your kidneys and dissolves into your urine, producing that distinctive bright or neon yellow color. This is different from the pale-to-amber spectrum you normally see. The effect is especially noticeable if you take a multivitamin or B-complex supplement, since these typically contain doses of B2 well above what your body can use in one sitting.
The color change can happen quickly. Because riboflavin is water-soluble and processed through the kidneys relatively fast, you may notice the bright yellow shift within a couple of hours of taking a supplement. It typically fades over the course of several hours as the excess clears from your system.
Is Bright Yellow Urine Harmful?
No. The bright yellow color is a harmless side effect of your body doing exactly what it’s supposed to do: getting rid of what it doesn’t need. There are no known side effects of excess riboflavin. Your body simply can’t overdose on it in any meaningful way because whatever it can’t use gets excreted. The clinical term for this yellow coloring is “flavinuria,” and it’s considered medically insignificant.
If anything, the neon yellow is a sign that your supplement is being absorbed and processed normally. It does not mean you’re wasting the entire vitamin. Your body took what it needed first, then discarded the rest.
Other Vitamins That Change Urine Color
Riboflavin is the main culprit behind bright yellow urine, but it’s not the only vitamin that can alter the color. Vitamins A and B12 can shift urine toward an orange or yellow-orange hue. Beta-carotene, which your body converts into vitamin A, can do the same. These shades tend to look warmer and darker than the fluorescent yellow that riboflavin produces.
Certain medications, foods like beets and asparagus, and artificial dyes in supplements can also change urine color. If you’re seeing red, brown, or blue-green tones that aren’t explained by something you ate or took, that’s worth paying attention to.
Bright Yellow vs. Dark Yellow From Dehydration
People sometimes confuse vitamin-induced bright yellow with the dark yellow of dehydration, but the two look quite different in practice. Dehydration produces a concentrated, deep amber or honey-colored urine because there’s less water diluting the waste products your kidneys filter. It often comes with other signs like thirst, dry mouth, or fatigue.
Riboflavin-colored urine, by contrast, looks almost neon or electric yellow. It’s vivid and fluorescent rather than deep and murky. You can be perfectly well-hydrated and still see this color after taking a B-complex vitamin. If your urine is bright yellow but you’re drinking plenty of water and feel fine, the likely explanation is riboflavin doing its thing.
Where Riboflavin Comes From
You get riboflavin from both food and supplements. Rich dietary sources include eggs, dairy products, lean meats, almonds, and fortified cereals and breads. Most people in developed countries get enough B2 from their diet alone, which is why the neon yellow effect usually shows up after taking a supplement rather than after eating a meal. Supplements concentrate the vitamin into a single dose that exceeds what your body can absorb at once, making the excess more noticeable in your urine.
If you stop taking the supplement, the bright yellow disappears within a day as your body clears the remaining riboflavin. Starting it again brings the color right back. It’s one of the most predictable and recognizable effects of any vitamin supplement.