Supplements for skin, hair, and nails

Collagen peptides (10–15 g + vitamin C cofactor) have 13+ trials supporting skin elasticity and joint comfort over 3+ months. Biotin shows hair effects only in actual deficiency; mega-dose biotin distorts thyroid lab tests. Niacinamide topically (and orally for some indications) is well-evidenced. Vitamin D3 if deficient improves skin barrier. Astaxanthin has small skin-photoprotection trials. Most 'beauty blends' rely on collagen + small amounts of cofactors — pay for the collagen, not the marketing.