Whether or not teens should be taking vitamins depends on how nutritious their usual diet is, on their lifestyle, and if they have special needs or vitamin deficiencies. For example, a teenager who skips meals frequently, tries out a new quick-loss diet every month, or thinks that fruits, vegetables and whole grains are gross, and that "real food" means potato chips, Coke and cookies, could be a candidate for nutritional supplements. In addition, if your teen suffers from an eating disorder, such as anorexia, bulimia or binge eating, it's likely that he or she is not getting anywhere near the nutrients needed to support the rapid growth and development of the adolescent years.
While teenagers need more calories than adults to fuel their changes and growth spurts, they should make sure to keep physically active so that the extra calories don't get stored as fat. The nutrients that teenagers need the most are proteins, carbohydrates, iron, fiber and calcium, and if they don't get these nutrients from their diet, you should speak to their doctor regarding which supplements they should take. In addition, if your teen has special medical needs, he or she may also need vitamins or mineral supplements.
If, on the other hand, your teenagers eat a variety of fresh fruits, vegetables, nuts, meat, fish, dairy products and whole grains, they probably are getting all of their nutritional needs met. If your teen experiences hair loss, you might want to check if you're dealing with a zinc deficiency. If your teen rarely spends time outside in the sunlight, the doctor could ascertain whether vitamin D levels are adequate. If your teens go through a vegetarian phase, you'll want to make sure that they're still getting enough protein, iron and vitamin B from other sources.