How much did they put in this?!

Temporarily. With daily use it goes clear again. Riboflavin is absorbed via saturable carrier-mediated transport (RFVT1/2/3 transporters) in the small intestine, then converted in the intestinal mucosa, liver, heart, and kidneys into its active coenzyme forms: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These flavin coenzymes are the functional forms your cells actually use as redox cofactors across dozens of flavoprotein-dependent reactions including the electron transport chain, beta-oxidation, the Krebs cycle, and MTHFR activity. When someone starts supplementing from a suboptimal baseline, those intracellular binding sites across multiple tissues are not yet saturated. The body's uptake capacity for free riboflavin exceeds what can be immediately incorporated into apoenzymes, but there is a ceiling. Any riboflavin in circulation above what can be bound gets filtered at the glomerulus, partially reabsorbed via saturable renal tubular reabsorption, and the remainder is excreted. Riboflavin is a naturally yellow-green fluorescent isoalloxazine compound, so even small concentrations in urine produce a vivid color.

After approximately 1-2 weeks of consistent daily dosing, tissue flavoprotein binding sites reach saturation. At that inflection point, urinary excretion plateaus relative to intake because a larger fraction of incoming riboflavin is being productively incorporated rather than remaining as free riboflavin in plasma. Efflux of free riboflavin into extracellular compartments is also regulated by the ABCG2 transporter (BCRP), which actively pumps unbound riboflavin out of cells; as tissue demand is satisfied, this efflux also diminishes. The net result is less free riboflavin reaching the renal filtrate, and the urine clears. Essentially, your body figures out how to more effectively use the vitamin after you get your levels healthy. This is also why simplifying it down into: if you see yellow in your pee, you are taking too much B vitamins. The pharmacokinetics are a lot more complex than that, and you need to reach steady state plasma levels to really assess things. We spent a lot of time formulating Infini-B, and many of the RDIs for B vitamins are just totally wrong.