NOW® Vitamin D-3 Extra Strength liquid supply this key vitamin in a highly absorbable liquid form. Vitamin D is normally obtained from the diet or produced by the skin from the ultraviolet energy of the sun. However, it is not abundant in food. As more people avoid sun exposure, vitamin D supplementation becomes even more necessary to ensure that your body receives an adequate supply. This extra strength product has ten times the D3 as our regular strength product.