Pro-Collagen Synthesis Super Antioxidant Serum. Medik8 CE-Tetra is a revolutionary ultra-stable vitamin C serum which helps combat sun-induced damage to the skin, trigger collagen synthesis, increase the skin’s elasticity and density as well as improve the skin’s texture and brighten the complexion. Designed for both men and women in their mid-20s to 40s. Regular vitamin C is very unstable and many typical vitamin C products are inactive by the time they are used on the skin, in some cases, they have become pro-oxidant and actually generate free radicals! Ground-breaking stability data on CE-Tetra guarantees real vitamin C activity and best of all, unlike L-ascorbic acid (regular vitamin C), C-Tetra will not exfoliate or irritate skin. CE-Tetra contains 7% Tetrahexyldecyl Ascorbate, the most potent and highly stable derivative of vitamin C available. CE-Tetra has been shown to have upto 4 times the potency L-ascorbic acid and other derivatives such as MAP.