Bright, neon or fluorescent yellow urine is most commonly caused by riboflavin (vitamin B2), found in many multivitamins and B-complex supplements. Riboflavin is water-soluble, so the body absorbs only what it needs and excretes the excess through the kidneys, giving urine a distinctive glowing yellow-green color. This is harmless and usually shows up within a few hours of taking a supplement, fading once the excess vitamin clears your system, typically within a day.
Other B vitamins and certain other supplements or medications can produce similar effects, and eating large amounts of foods with artificial dyes can occasionally tint urine as well. Mild dehydration can also make urine appear more concentrated and vividly yellow, though that shade tends to be darker amber rather than bright neon — increasing water intake usually normalizes the color quickly.
If you haven't taken any vitamins or supplements recently and your urine is neon yellow, it's worth thinking back over the last day or so at what you ate or drank, including energy drinks, which often contain high doses of riboflavin. In the vast majority of cases, neon yellow urine on its own is not a sign of a medical problem.
That said, pay attention to accompanying symptoms. If the color is accompanied by a strong or unusual odor, pain or burning during urination, cloudiness, blood, fever, back pain, or if the bright color persists for several days despite stopping supplements and drinking normal amounts of fluids, it's a good idea to check in with a doctor, since these could point to a urinary tract infection, kidney issue, or liver problem. Certain medications (like some antibiotics or chemotherapy drugs) can also change urine color, so review any new prescriptions with your pharmacist or doctor if you're concerned. Generally, though, neon yellow is one of the more benign and easily explained urine colors.