Because B-complex vitamins are water-soluble, excess amounts are typically filtered out through urine, making toxicity rare under normal dietary conditions. However, consuming massive, unmonitored megadoses via supplements can trigger distinct adverse side effects. Excessive niacin (B3) can cause severe skin flushing, itching, and gastrointestinal upset, while high levels of pyridoxine (B6) over long periods can lead to nerve toxicity, numbness, or tingling in the extremities. High doses of other B vitamins can occasionally cause nausea, vomiting, or blurred vision.