The general rule — and its exceptions
Because water-soluble vitamins dissolve in water, the body excretes the surplus in urine rather than storing it (the main exception being B12, stored in the liver). That makes them harder to overdo than the fat-soluble vitamins A, D, E, and K. But 'harder' isn't 'impossible' — several water-soluble vitamins have a Tolerable Upper Intake Level for good reason.
The water-soluble vitamins that DO have upper limits
- Vitamin B6 — UL 100 mg/day; chronic high doses can cause sensory nerve effects (see [vitamin B6 and neuropathy](/learn/vitamin-b6-intake-and-neuropathy)) [1].
- Niacin (B3) — UL 35 mg/day for supplemental niacin, based on flushing; gram-level doses can affect the liver (see [niacin high-dose safety](/learn/niacin-high-dose-safety)) [2].
- Folic acid — UL 1,000 mcg/day; high intake can mask a [B12 deficiency](/learn/folate-intake-and-b12-masking) [3].
- Vitamin C — UL 2,000 mg/day; above it, digestive upset and diarrhea are common, and very high intakes may raise kidney-stone risk in susceptible people (see [vitamin C upper limit](/learn/vitamin-c-intake-upper-limit)).
The ones with no upper limit
Thiamin (B1), riboflavin (B2), pantothenic acid (B5), B12, and biotin have no UL because high intakes haven't been linked to toxicity — though biotin can still distort lab tests. 'No UL' means limited evidence of harm, not a license to megadose (see why some nutrients have no upper limit).
Practical guidance
- Don't assume water-soluble = unlimited. Check B6, niacin, folic acid, and vitamin C totals across products.
- Watch stacking: B-complex, energy, and 'beauty' products can push B6 or niacin higher than you'd expect (see [stacking safely](/learn/supplement-stacking-safety)).
- Megadoses mostly produce expensive urine — more isn't better.