Keep in mind that some ingredients found in dietary supplements are added to a growing number of foods, including breakfast cereals and beverages. As a result, you may be getting more of these ingredients than you think, and more might not be better. Taking more than you need is always more expensive and can also raise your risk of experiencing side effects. For example, getting too much vitamin A can cause headaches and liver damage, reduce bone strength, and cause birth defects. Excess iron causes nausea and vomiting and may damage the liver and other organs.
Be cautious about taking dietary supplements if you are pregnant or nursing. Also, be careful about giving them (beyond a basic multivitamin/mineral product) to a child. Most dietary supplements have not been well tested for safety in pregnant women, nursing mothers, or children.
from the Mayo Clinic: Nutritional supplements are meant to complement a healthy diet, not replace it. If you're generally healthy and eat a wide variety of foods, you probably don't need nutritional supplements. In certain cases, however, fortified foods and dietary supplements may be useful in providing one or more nutrients. For example, adults age 50 or older may not get enough vitamin B-12 in their diets and may need fortified foods or a nutritional supplement.
You also may want to consider a nutritional supplement if you don't eat enough healthy foods or don't eat a healthy variety of foods — especially if you have an underlying medical condition. Talk with your doctor or a dietitian about whether nutritional supplements might be appropriate for you. Be sure to ask about possible side effects of nutritional supplements you're considering — and if they have interactions with medications you take.