What percentage of vitamin C is most effective for the skin?

Studies show that the amount of ascorbic acid one needs to achieve results regarding skin repair is roughly 10% daily. However, applying a topical that contains 10 to 15% l-ascorbic acid once a day is much less efficient than applying a 5% l-ascorbic topical twice a day-- for two reasons. Firstly, your skin will utilize the amount it needs at any given time and discard the rest (the same goes when taking supplements internally--your body excretes what it cannot use, so taking 500 mg twice a day gives you more usable Vitamin C than taking 1,000 mg all at once). Secondly, applying too much Vitamin C at any given time can irritate the skin--it is an acid after all. Our Vitamin C+E+Ferulic Serum contains 5% l-ascorbic acid and roughly 2% C derivatives. Twice daily applications will allow Vitamin C to prevent and correct hyperpigmentation and act as an antioxidant during the day while maximizing repair processes like collagen synthesis at night. Vitamins C+E+Ferulic Serum contains l-ascorbic acid--the only Vitamin C that actually works to improve skin quality. Although most skin can tolerate l-ascorbic acid some skin, perhaps due to sensitization issues, cannot, and this is where derivatives come in. Oil-soluble Vitamin C derivatives are more easily tolerated by sensitive skin than water-soluble l-ascorbic acid because they are more stable, and they also penetrate very well. When they penetrate and convert to ascorbic acid they deliver the same benefits as ascorbic acid. Indeed, derivative technology has come a long way--studies show that tetrahexyldecyl ascorbate is almost as effective as ascorbic acid itself. In addition, using a range of derivatives with different conversion rates helps to regulate l-ascorbic acid delivery to the various layers of the skin where it is needed; a well-formulated Vitamin C serum can function like a slow-release Vitamin C capsule. This is very important for those with sensitive or symptomatic skin, for whom we recommend our C-Therapy Serum, formulated with four powerful and effective Vitamin C derivatives.