Your questions about vitamin C answeredWhat is the best vitamin C serum?My top choice is the Olay super serum, a mid-range priced, all-rounder product that significantly improves texture with regular use. Fast-absorbing, pleasantly scented and multi-purpose, it’s suitable for even the most sensitive skin. If you’re shopping on a budget, the Boots vitamin C + hyaluronic acid serum is a great pick, not least for its low price tag, but also its matte finish and ability to balance combination skin, while luxury lovers will enjoy Kiehl's powerful-strength line-reducing concentrate, particularly for its brightening and skin-blurring finish. How I tested the best vitamin C serums Over the course of nearly two months, I introduced a selection of vitamin C serums into my morning routine, applied to clean, dry skin, followed by moisturiser or SPF, sometimes both. Suitability for sensitive skin: Vitamin C isn’t always well tolerated, and irritation can be common, so I factored in how well each serum reacted with my sensitive skin, and whether it irritated my acne or rosacea.Texture and performance: I examined how fast it absorbed, the finish it left behind and how well it layered beneath make-up during the day. Routine compatibility: Any serum can’t just be used on its own; it’s important to ensure it also plays well with the rest of my routine, including my moisturiser, so I kept a close eye on whether it caused any pilling or reacted with any prescription products I use for my acne.What is vitamin C?Dr Haus describes vitamin C as a “powerful antioxidant” that plays a protective role for the skin against UV exposure, pigmentation, and dullness. “Its popularity stems from its ability to protect, repair and enhance the skin's overall quality, making it the superhero in my patients’ routines,” he says. When should vitamin C be applied? Dr Haus advises applying any vitamin C product in the morning on clean, dry skin, before applying your usual moisturiser and SPF. “Morning application maximises its antioxidant protection, getting you maximum protection from UV, pollution, and other environmental stressors,” he notes. Are there any ingredients vitamin C is not compatible in the rest of your routine?“When used at the same time, vitamin C and retinol can at best become less effective, or at worst irritate the skin,” says Dr Haus. This is why he recommends applying your vitamin C in the morning and your retinol in the evening, to get the benefits of both and minimise irritation.Are there different types of vitamin C in skincare? There are a few different varieties of vitamin C within skincare – L-ascorbic acid (or just ascorbic acid) is the most common and works well for oily or normal skin. However, those with sensitive skin may find it harsh. For more sensitive skin, other variations, including sodium ascorbyl phosphate and tetrahexyldecyl ascorbate (THD) are less irritating.Who should be using vitamin C serums? Most skin types can benefit from a vitamin C serum, unless you have an allergy to triethanolamine. If you have sensitive skin or are using vitamin C for the first time, it’s best to start with a lower concentration (under 10 per cent) or mix it with a moisturiser for a gentler introduction. Alternatively, you could opt for a moisturiser that already contains vitamin C.What does the concentration of vitamin C mean? Similar to alcohol, vitamin C serums will have a percentage concentration of either L-ascorbic acid or other types of vitamin C. Ideally, you want to start at 10 per cent or less, and ease your way up the scale to see what works best with your skin.The general consensus is that anything above 20 per cent is too potent and can lead to irritation without increasing any skincare benefits.How to store vitamin C serumsThe general rule of thumb is to store Vitamin C serums in a cool, dry place with the lid tightly sealed. If you notice a rancid smell or the colour of the product changes to a deep yellow, or a dark or cloudy colour, it may have oxidised, so you can’t use it.Which other ingredients make for a great vitamin C serum? The unstable nature of vitamin C serums means that other antioxidants and vitamins are included to help balance the products and provide additional benefits. Core ingredients include vitamin E and ferulic acid to protect against cell damage, reduce inflammation and brighten skin tone.Why you can trust IndyBest reviewsLouise Whitbread is a beauty writer and editor with years of experience reviewing skincare. In addition to interviewing dermatologists for skincare tips, such as how to reduce acne scarring, and insights into skincare ingredients and their benefits, Louise considers all skin types when reviewing products. From night creams to cleansing balms, Louise always factors in sensitivity, and related skin concerns such as redness and hyperpigmentation. What’s more, as someone who experiences rosacea and hormonal acne herself, Louise was able to test these vitamin C serums on her sensitivity-prone skin. The products listed in this article are her honest, tried-and-tested recommendations. Meet the expertDr Ariel Haus: Harley Street dermatologistLooking for more skincare recommendations? Read IndyBest’s review of the best retinol serums and creams