Vitamin C actually enhances the effectiveness of Retin A. Dermatologists are divided on the topic of AHA & BHA rendering Retin A or retinol products ineffective. I can tell you I've been using Retin A for over 20 years now and I regularly apply after AHA & BHA & Vitamin C and have never had any adverse effects from doing so. In fact, when I do apply my Retin A over any of the above, I wake up with even smoother skin. Paula Begoun and her team agree, as well as many dermatologists, that it's beneficial to use these ingredients together.
My opinion is to try it for yourself and see the results and then decide. If your skin can tolerate layering potent products, then go for it.
I wouldn't for a couple of reasons. Most importantly, it contains Vitamin C AND AHAs/BHAs (glycolic and salicylic acid), which are all acidic. The optimal pH for retinol is around 5.5-6.5 (normal skin pH), and the low pH of the above ingredients will *inactivate* your retinol. Plus, the higher pH of the retinol will make the Vitamin C less effective. You could theoretically use both at night by spacing them out by 45 minutes to an hour, to ensure that they are each absorbed at the right pH (retinol at a higher pH, then Vit C at a lower pH). However, I wouldn't suggest using AHAs/BHAs and retinol at the same time of day, since they can both be irritating.
I think the reason they suggest using this product at night is because the AHAs/BHAs can cause photosensitivity (but so can retinol)--if you use it during the day, but sure to use a physical sunblock over it (in this case, both the serum and the sunblock will work more effectively if you use physical rather than chemical sunscreen). If you're set on using retinol at night, I'd use this in the morning, then a fat-soluble, non-acidic Vitamin C product containing magnesium ascorbyl phosphate, sodium ascorbyl phosphate, or ascorbyl glucoside.
answered 3 years, 4 months ago