Vitamin B complex supplements are generally safe for most individuals when taken as directed. However, some people may experience mild side effects, especially when starting a new supplement or taking higher doses than recommended. Common side effects may include nausea, stomach upset, and diarrhea. Also, vitamin B6 toxicity, which usually results from overdosing, may cause peripheral neuropathy with long-term intake.Vitamin B complex supplements are generally safe for most individuals when taken as directed. However, some people may experience mild side effects, especially when starting a new supplement or taking higher doses than recommended. Common side effects may include nausea, stomach upset, and diarrhea. Also, vitamin B6 toxicity, which usually results from overdosing, may cause peripheral neuropathy with long-term intake.