Testing skincare products is an occupational hazard. In the past, I’ve been lucky. I’m relatively non-reactive (I used to claim). But, boy, have I been humbled.

I had polished off the last of my morning serum, the Major Fade Hyper Serumby #PillowtalkDerm, which I highly recommend. I swapped in a vitamin C serum from a reputable brand that came highly recommended by derms and other editors. Also, Kaia Gerber uses the full line, non-sponsored. However, I wasn’t really paying attention to this serum’s label, which read: 25% L-ascorbic acid.

You may be thinking, Careful with that! For those uninitiated, L-ascorbic acid is the active form of the antioxidant vitamin C, commonplace in today’s “brightening” serums. According to dermatologists, L-ascorbic acid has long been credited as the most effective form of vitamin C, the gold standard, even. The problem? It’s unstable and can, in high concentrations, cause irritation or an inflammatory reaction.

While this particular vitamin C serum didn’t actually burn or sting on contact with my skin, just two days after using it, I woke up with my right eye swollen shut. Picture that scene in Hitch when Will Smith is sucking Benadryl with a straw. The swelling quickly progressed to red, dry, scaly skin around my eyes, and perioral dermatitis, which I self-diagnosed after Googling “white bumpy rash around mouth.” (Note: If you have an allergic reaction to a skincare product, see a medical professional right away.)

My doctor prescribed me a tapered oral Prednisone. She and my derm both instructed discontinuing all skincare immediately. I rinsed out the vitamin C serum and sorted it in the recycling. It was a $140 bottle, so literally, money down the drain.

My swelling went down fast. After about a week, using nothing on my face other than the rogue spritz of the Tower28 SOS Daily Rescue Facial Spray with hypochlorous acid, my skin bounced back to baseline. But I had lingering questions. Why did this vitamin C serum cause such a severe reaction? And, must I now give up topical vitamin C forever? Here’s what I gathered.

Mind The Concentration

I want to be clear: This inflammation reaction was not the fault of this specific serum nor vitamin C as an ingredient; this was a major user error. My primary mistake was, you guessed it, not reading my label. The serum was brand new, and thoughtfully packaged in an airless bottle as opposed to a glass dropper that may have exposed the formula to light or air, so I’m confident that instability was not the issue. Simply, its 25% concentration simply proved too high for my sensitive skin. According to dermatologist Dr. Shereene Idriss who talks about vitamin C a lot on her YouTube channel, sensitive skin types would be advised to use a concentration of L-ascorbic acid less than 10%. “Anything more than 10% could irritate you,” Dr. Idriss explains.

Consider The pH Scale

Though it’s an antioxidant, vitamin C is acidic, especially those L-ascorbic acid-based formulas. “The problem with L-ascorbic is that it’s highly unstable and in order to be stable, it has to be put at a very low pH, which tends to burn and irritate sensitive skin,” explains dermatologist Dr. Whitney Bowe. For context, our skin has a baseline pH of 5.5. In contrast, L-ascorbic-based vitamin C serums are generally formulated at a pH of 3.5 or lower, which is comparable to that of an at-home chemical peel.

With hindsight, had I understood this elementary pH equation, I would have treated the vitamin C serum less like a simple, everyday serum to be slapped on willy-nilly, and more like a peel pad that could burn my skin like a beef carpaccio.

Be Careful Of Over-Exfoliation

Which leads me to my next problem: Prior to implementing the high-concentration active vitamin C serum, I was regularly using retinol at night and exfoliating my skin twice a week. This is fine, but it’s likely that the addition of the high-strength L-ascorbic acid pushed my barrier to its reactive break. My colleague Jacqueline Kilikita reported on the problem with exfoliating acids in skincare. In it, she speaks to the dangers of layering up the often-sexy exfoliating acids that promise a glow, but can compromise the skin barrier, especially if you’re sensitive.

Again, the L-ascorbic acid itself is not the issue, potential over-exfoliation is. In the future, I would take a more conservative approach to any vitamin C introduction: patch testing a serum before putting it on my face, always following with a hydrating moisturizer and cycling out all other exfoliating acids in my routine.

Try Inactive Forms Of Vitamin C

I’m not writing off L-ascorbic acid, because it works (just ask anyone who uses C E Ferulic and has fabulous, glowing skin — there’s a lot of them). However, given that my sensitive skin reacted negatively to it, Dr. Bowe offers an alternative: “I’m really excited about the newer, more stable [vitamin] C derivatives, like tetrahexyldecyl ascorbate.” Dr. Idriss has also mentioned tetrahexyldecyl ascorbate as a great vitamin C option for those with sensitive skin. “It’s oil soluble as opposed to L-ascorbic acid, which is water soluble,” Dr. Idriss explains. “It can be formulated at a pH of around 5, so it’s not too acidic, and many skin types can tolerate it.”

While tetrahexyldecyl ascorbate is an inactive form of vitamin C — which means it has to be converted to L-ascorbic acid within the skin before it can get to work — it offers the same benefits as the active form. “It’s a skin brightener, it’s also a collagen booster, and it has the photo-protective antioxidant properties of pure vitamin C,” says Dr. Idriss. By that logic, it functions the same as L-ascorbic acid, just gentler.

It’s Okay To Not Use Vitamin C At All

Skincare is ultimately personal. This isn’t a takedown of vitamin C. However, there are some dermatologists who don’t love it. Consultant dermatologist Dr. Anjali Mahto has previously said, “I’m not regular about using a vitamin C in my own skincare routine, and personally, I could take it or leave it.” Here’s why: “I don’t like products being layered on my patients’ skin, either,” Dr. Mahto explained. “The more variables I put into my treatment plan, the more likely it is that you will get peeling, irritation, and sensitivity.” Instead of vitamin C, Dr. Mahto is more likely to recommend azelaic acid, which also helps with pigmentation and evening out skin tone.

Personally, I’m still on a very lean routine: I’m using a gentle oil-based cleanser, mostly to remove my makeup, and then a moisturizer and sunscreen. My whole scary vitamin C reaction and the research that followed made me think a lot about the reporting of Jessica Defino at The Unpublishable. She talks a lot about how our skin is a self-regulating organ and posits that skincare products often create our problems, doing more harm than good, especially at an industry-large scale. In the case of vitamin C serum and my recently-traumatized skin, for now, I’m better off without it.

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