B vitamin complex supplements are generally safe, but high doses may cause side effects. Common side effects can include digestive upset, such as nausea, vomiting or diarrhea. This is especially true with high doses of niacin (B3). Additionally, high doses of certain B vitamins, such as B6, can cause neurological symptoms. These include tingling or numbness in the hands and feet. Follow the recommended dosage on the label and talk to your doctor if you experience side effects.