The best way to get the vitamins and minerals your body needs are through eating a nutritionally balanced diet.  However, sometimes a supplement may be necessary:

• You’re over 65 or older. As you get older, health problems can contribute to a poor diet making it difficult to get enough vitamins and minerals. Twenty percent of adults over 60 and 40% over 80 don’t absorb enough vitamin B12 from food; therefore as you get older it is necessary you take a multivitamin.
• Postmenopausal woman. Need both calcium and vitamin D, these vitamins have  shown to protect against osteoporosis.
• You eat poorly. If you find you do not eat a well balanced diet, taking a multivitamin can be useful. Although watch for supplements that provide a “megadose”.  Choose a multivitamin supplement that provides about 100% DV of all the vitamins and minerals, with the exception of calcium. Otherwise, the tablet would be too large to swallow.
• When you are on a very low calorie diet.  If you are eating fewer than 1,000 calories a day; which is not advisable, you may need a multivitamin.
• You smoke. Tobacco decreases the absorption of many vitamins and minerals, including B-6, vitamin C, folic acid and niacin.
• You drink excess alchol. Long term excessive alchol consumption can reduce the digestion and absorption of thiamin, folic acid, vitamins A, D and B-12. It also affects the minerals zinc, selenium, magnesium and phosphorus.
• When you’re pregnant or trying to become pregnant. During this time you need more calcium, folic acid and iron. Food folate is not as well absorbed as synthetic folic acid.
• If you have special dietary restrictions.  If you’re vegetarian and eliminate all animal products from your diet, you may need additional vitamin B-12.  
• Your body can’t absorb nutrients properly.  If you have a disease of your gall bladder, liver, intestine, pancreas, or if you have had surgery on your digestive tract, this may result in the inability to digest and absorb nutrients properly.

Dietary supplements can cause problems to nutrient excess, imbalance or adverse interactions with certain prescription drugs.  High doses of single nutrients may also interfere with another nutrient. High doses of vitamin E interfere with vitamin K and enhance the effect of coumarin anticoagulant drugs.  High doses of calcium inhibit iron absorption. Folic acid can mask the signs of B-12 deficiency and may also interfere with anticonvulsant medications.  Zinc supplementation may decrease HDL cholesterol.