- Caused by deficiency of niacin (vitamin B3)
- Causes symmetric hyperpigmented rash that looks like a sunburn, on sunexposed skin ("Casal necklace")
- Also: red tongue, diarrhea, vomiting, insomnia, anxiety, disorientation, delusions, dementia, and encephalopathy
- May be caused by carcinoid syndrome or long-term isoniazid
