Persistent fatigue is rarely caused by a single vitamin. Common contributors include low iron stores (ferritin), vitamin B12, folate, and vitamin D. B12 deficiency can cause fatigue with or without anemia and may also present with tingling, cognitive changes, or tongue soreness. Iron deficiency can lead to exertional fatigue, shortness of breath, palpitations, and hair shedding. Vitamin D insufficiency may contribute to muscle aches and reduced well-being but is not the sole explanation for all fatigue. Instead of random supplementation, consider targeted testing (CBC, ferritin, B12, folate, vitamin D, TSH) and clinician-guided dosing.