If you've been tested and found to have a vitamin deficiency, that's the time to discuss a supplement regimen with your doctor — though, again, there's no guarantee of if or when you'll see a difference. "From what researchers know about the internal path of ingestible supplements, the body often funnels the nutrients where they are most needed," Dr. Nussbaum explains. "That may be the skin, hair, or nails, but it could also be internal functional organs, which may not show direct physical results. Generally speaking, skin-cell turnover differs depending on age, skin health, and topical routine. It could likely take anywhere from a few weeks to several months to see changes in hair, skin, or nail heath depending on those factors."