Peter Thomas Roth Potent-C Vitamin C Power Serum In case you were wondering why the Peter Thomas Roth Potent-C Vitamin C Power Serum ($98) is worth the buy, it's because it contains 20 percent THD ascorbate — a form of vitamin C that can be up to 50 times more powerful than its counterpart — to help visibly brighten dark spots and even out the skin. Image Source: sephora.com Peter Thomas Roth Potent-C Vitamin C Power Serum $98 from sephora.com Buy Now