If you're taking a multivitamin, it's not a good idea to supplement with extra individual vitamins. High levels of any single nutrient can lead to overdose and toxicity in the body, especially with the fats soluble Vitamins A, D, and E. For example, a Vitamin A overdose can lead to dizziness, blurred vision, and headaches. A Vitamin D overdose can lead to nausea, fatigue, a loss of appetite, and kidney stones. And finally, a Vitamin E overdose can lead to hypertension, severe fatigue, and muscle weakness. Multivitamins can be supplemented by things like probiotics or fish oils, but not extra vitamins supplements. This way you can avoid a vitamin overdose and toxicity in your body. Remember that the definition of a supplement is something that completes or enhances something else. So, when taking vitamin supplements you should use it as that. As a supplement to real food, not as a quick fix or to replace it.