I’ve been ordering Dr. Brenner C Serum with Vit. E, Ferrulic & Hyaluronic Acid 1 oz. for approx. 5 years, from 2018-2023. I’ve gone through approx. 8-12 bottles, opening and using up one bottle at a time. Never storing any, I always keep my bottle in the refrigerator. Of the 5-6 brands of C Serums I’ve used, Dr. Brenner always stayed clearest longest. Most oxidize at approx. 1-3 months. Not Brenner’s- that is, until 2023. Beginning in 2023 the formula has begun to oxidize as quickly as at 1 month, even when kept in the fridge. This has been true for the last 3 Dr. Brenner c Serum bottles I’ve purchased. For this reason I am leaving a 3-star review, and have moved on to experimenting with other C Serum brands. I hope the Dr. Brenner quality improves to what it has been in years prior to 2023. Thank you.