Vitamin B-complex supplements are generally considered safe when taken in appropriate doses. However, excessive intake of certain B vitamins can cause side effects. For example, high doses of vitamin B6 may lead to nerve damage, while large amounts of vitamin B3 can cause flushing or skin irritation. B-complex supplements may also interact with medications or other supplements. It is crucial to follow recommended dosages and consult a healthcare professional before starting any new supplement regimen, especially if you have underlying health conditions or are taking medications.