Mild flushing or upset stomach may occur. Contact your doctor or pharmacist if any of these effects persist or worsen but most effects are usually temporary and will go away when your body adjusts to the vitamin. You will also experience the color of your urine to have a bright yellow color after taking a B-complex. Some often also describe the color as neon yellow. This comes from the B2 vitamin, also known as riboflavin. Do not be alarmed as this is completely normal!