If you start experiencing signs and symptoms of a specific vitamin deficiency (a good place to start looking for these is your fingernails and hair), see your doctor and let he or she know your concerns. Based on their observations, the doctor will either recommend starting with a daily multivitamin or will order blood work. The best and most sure fire way to diagnose a vitamin deficiency is to have the appropriate lab tests i.e. blood work. Then, depending on the severity of the vitamin deficiency, you may be prescribed supplements or told to purchase some over the counter. Not all vitamins are created equally, so some symptoms will be severe and may require immediate medical attention while others are mild and can be treated conservatively. Don't be your own physician when it comes to vitamins as many have harmful interactions with one another and/or can cause more harm and good. I hope this is helpful.