Multivitamins often provide 100 percent of the daily value for most, if not all, of the B vitamins, making it unlikely you would also need to take a B-complex vitamin. The B vitamins are water-soluble, meaning that consuming more than you need of these vitamins results in the extra being excreted in your urine. Taking too much of certain B vitamins, however, can cause toxicity symptoms, so check the nutrition facts labels to make sure the combined supplements don't exceed the tolerable upper intake level for any of the B vitamins.
Nutrient of Concern
Two B-vitamins have toxicity concerns -- niacin and vitamin B-6 -- but taking both a multivitamin and a B-complex vitamin will only put you near the tolerable upper intake level for niacin. The recommended dietary allowance for vitamin B-6 for adults is 1.3 milligrams, compared to an upper limit of 100 milligrams. The RDA for niacin, however, is 16 milligrams per day for men and 14 milligrams per day for women, and the upper limit for adults is only 35 milligrams per day. Thus, taking two different supplements that each contain the RDA will put you almost at the limit, not counting any niacin you may have gotten through food. Consuming too much niacin can cause nausea, vomiting, blurred vision, liver problems, as well as high-blood sugar.