Vitamin B2 (Riboflavin)
Supplements · Vitamins · Supplement, Typed Supps
The one that turns your urine bright yellow when you take more than you need.
Stage by stage
- Intake (Tablet): Absorbed via a saturable transport system in the small intestine.
- Small intestine (Saturable transport): Very high single doses absorb proportionally less due to this saturation.
- Bloodstream (Carried to cells): Carried in blood to tissues throughout the body.
- Energy pathways (Activates other B vitamins): Used as a cofactor in energy production and to help activate B6 and folate.
What the evidence supports
- Correcting a documented deficiency — strong evidence. Clear, though isolated deficiency is uncommon where dairy is eaten.
- Reducing migraine frequency — moderate evidence. Several trials at 400 mg daily found fewer migraine days, and it appears in some headache guidelines as an option. The effect takes two to three months to appear.
Typical dose
1 to 3 mg daily covers ordinary supplementation and is what a multivitamin supplies. The migraine trials used 400 mg daily, which is a different proposition entirely and needs two to three months before you can judge it.
Upper limit: No UL has been set. No adverse effects have been reported from high intakes from food or supplements, including 400 mg daily for at least three months. Absorption in the gut is limited, which is probably why.
Timing
With food, standard dosing.
Time to effect
- Weeks: Levels normalize if deficient.
Medication interactions
- Phenothiazine antipsychotics and tricyclic antidepressants: Chlorpromazine, imipramine and amitriptyline inhibit flavokinase, the enzyme that turns riboflavin into its active coenzyme forms, and can lower riboflavin status. The work behind this is animal and laboratory rather than clinical. Worth knowing if you take one long term and eat little dairy; not a reason to alter either the drug or the supplement. (moderate)
- Probenecid: Reduces riboflavin absorption from the gut and increases urinary loss. The practical effect at ordinary intakes is small. It is worth saying plainly that riboflavin is unusual here: the NIH fact sheet records no clinically relevant medication interactions for it, and nothing on this short list rises to that level. (moderate)
Common myth
“Bright yellow urine means something is wrong.” In reality: It’s simply excess riboflavin being excreted — a harmless, expected sign the water-soluble vitamin is working.
Food sources
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