Vitamin B supplements, especially those containing riboflavin (vitamin B2), often cause bright yellow or neon colored urine. This happens because excess riboflavin is water soluble and is excreted through urine when the body has Vitamin B supplements, especially those containing riboflavin (vitamin B2), often cause bright yellow or neon colored urine. This happens because excess riboflavin is water soluble and is excreted through urine when the body has Why is My Pee Bright Yellow? Yellow Urine Causes & Symptoms Why Is My Urine Yellow After Taking Vitamin B? Glutathione: What It Is, Why You Need It, How You Can Increase It The Amino Company