"Not all Vitamin C products are created equal," cautioned Dr Alexis Granite, Kiehl’s Dermatologist. "It can be a very tricky ingredient to use in skincare for a few reasons. Firstly, light and air can cause it to break down. This means that even if you start off with a great strength in a product, over time, the opening and closing of the packaging, and even the translucency of the packaging itself can deteriorate its efficacy. It’s also hard to get the dosage of Vitamin C just right, as it’s water soluble." Luckily, Kiehl's has come up with the solution to all these problems. "To keep it fresh and potent, Powerful Strength Line Reducing Concentrate is in a tinted opaque bottle to prevent light damage, and a pump to stop it being exposed to air. It was also a very long and laborious process to formulate it in a way that maintains a strong efficacy profile with no irritation," explained Dr Granite. "We have a strength of 12.5% Vitamin C in this new formulation, which will deliver all the results you need, but still be kind to skin."