The dosage of B vitamins completely depends on your current diet and lifestyle. In general, B-Complex vitamins are considered to be completely safe for most people, as long as they are taken in a sensible dose that suits their lifestyle. You are advised to avoid high doses of B vitamins as this could result in unwanted side effects, particularly with the use of Niacin (B3) for improving cholesterol and Pyridoxine (B6) as they could cause flushing or damage to the liver or nerves.
Healthy, balanced diets
Those who eat a healthy, balanced diet should gain adequate levels of B-Complex vitamins through food sources alone. Animal proteins like fish and chicken and organ meats such as liver are practically the only sources of vitamin B12. On the other hand, leafy green vegetables and legumes are great sources of folic acid (B9). Different foods are high in different types of B vitamin, which is why it's best to get a wide variety of foods throughout the day in order to avoid deficiency. If you feel that your diet is not quite varied enough, then you may need a lower dose of B-Complex vitamins.
Vegetarian or vegan diets
Those who are strictly vegetarian or vegan may find it difficult to gain a full range of B-Complex vitamins though diet alone. These types of restricted diet may find difficulty particularly with vitamin B12 as it is only found in animal proteins. You may be recommended a moderate potency vitamin B-Complex supplement plus an extra 100-500mcg of vitamin B12 daily. Those who avoid all meat and dairy products are at high risk of deficiency.
High carb diets
Vitamin B1 works mainly to produce the enzymes that burn carbohydrates for energy. High carb and high sugar diets will quickly use up these enzymes, meaning that your intake of vitamin B1 may need to be enhanced.
If you use certain medications then you may be at risk of lower B vitamin levels and may need an extra vitamin B boost from supplements. Many over-the-counter medications could interfere with the absorption of certain vitamins, such as medications for heartburn and reflux (all antacids), oral antibiotics and oral contraceptives.