Short answer: Niacin can cause flushing — warmth, redness or tingling across the face, arms and chest — which is harmless but can be uncomfortable, a direct, honest disclosure worth expecting.
Why "harmless but uncomfortable" is a careful, honest qualifier
This distinguishes a genuinely common, expected niacin effect from a dangerous one — worth understanding this distinction rather than being alarmed by the sensation itself.
Where to find more detail on this specific effect
The NIH ODS niacin fact sheet covers recommended intakes, upper limits and the flushing effect specifically — a genuine, independent source worth checking.
💡 Worth knowing: amino acids may cause mild digestive upset in some people too — a second, separate disclosed effect worth knowing about alongside flushing.