Catherine,
Taking a vitamin certainly can help for many forms of hair loss. The extra Biotin, Niacin, B-6, B-5, Paba, and Inositol can help. However, a full multivitamin contains Vitamin D, Vitamin A (no more than 3500 IU!), Vitamin E, Zinc, Copper and Silica, all of which are necessary for hair growth. Unless you have been on a drastic diet (those cleansing teas count!) or eat unhealthfully vitamins are only marginally helpful. (A quick way to look for nutritional defiencies affecting hair is to look at your nails. If they are thin, brittle or grow very slowly, there may be a problem.)
However, if your hair loss is genetic, related to medication you take, or hormonal you will need to do more than just take a vitamin. I have found that Dr. Lewenberg's Formula is effective treatment for most forms of hair loss. To see if it is right for you, please fill out the patient questionnaire . Remember, many conditions cause hair loss and the proper treatment varies by condition.