"In general taking large doses of water-soluble vitamins has not been found to have a deleterious clinical effect. However niacin (vitamin B3) excess can be problematic, usually when greater than 100 mg niacin is taken per day. To put this in context the recommended daily allowance (RDA) in the USA is 35 mg."
"These large doses may cause transient symptoms, most commonly flushing of the skin, although nausea, vomiting and diarrhoea have also been seen. Hepatic toxicity has been seen in rare cases, usually with megadoses of 1 gram, or more, per day."