Are Your Serums Conflicting? A Self-Check

If your skin stings right after you apply your serums, or flakes and reddens more than usual, the serums may be stacking irritation. If retinol and an exfoliating acid are both in your routine, use them on different nights rather than layering them on the same night. Among the pairings covered here, the only one where one ingredient was shown to break down another was benzoyl peroxide with tretinoin, and that was measured in a container, not on skin (Martin et al., 1998). For the other pairings, check first whether the irritation is adding up.

Can serums really cancel each other out?

When tretinoin was mixed with benzoyl peroxide and left under light, more than half of the tretinoin broke down in about 2 hours, and 95% within 24 hours (Martin et al., 1998). In the same test, adapalene stayed stable with or without light. Because this was measured in a mixture, it doesn't show the same speed on a face, and retinol wasn't part of the test. Tretinoin is prescription-only; benzoyl peroxide is sold over the counter, and adapalene is over the counter in the US but prescription-only in Korea. If you've been prescribed tretinoin and also use benzoyl peroxide, don't mix the two on your skin. Ask the doctor who prescribed it when to apply each one.

Can you layer retinol and exfoliating acids on the same night?

Retinoids are known to loosen the outer layer of skin and raise transepidermal water loss (TEWL). Glycolic, lactic and salicylic acid also loosen and lift dead surface cells. We couldn't find a comparison study that measured irritation in people layering retinol with these acids at once. Use retinol and glycolic, lactic or salicylic acid on different nights rather than layering them together. Splitting nights is a common approach; it hasn't been tested in a trial.

Is vitamin C really off-limits with niacinamide?

A common worry is that niacinamide turns into niacin when it meets acidic products, and that this makes the face flush. The conversion of niacinamide to niacin is known to speed up only with prolonged heating at high temperature in strong acid or base. We couldn't find a study that measured flushing in people applying vitamin C and niacinamide together. Niacinamide doesn't need to come out of your routine just because you use vitamin C that day. For pairing it with exfoliating acids, see whether niacinamide and exfoliating acids can share a routine.

In short, check whether irritation is stacking up first

Under light, benzoyl peroxide breaks tretinoin down. Retinoids are known to loosen the outer layer of skin. Mixed with benzoyl peroxide and exposed to light, 95% of tretinoin broke down within 24 hours (Martin et al., 1998). The pairing shown to break down here was two acne medicines; for the rest, the thing to check is whether irritation is stacking up. If stinging, flaking or redness keeps coming back after you apply, drop one active serum and add new serums one at a time. Change several at once and you won't be able to tell which one caused it. For how many steps a routine actually needs, see why you don't need a 10-step routine.

What you can try today

• Check the ingredient lists of tonight's serums for retinol and for glycolic, lactic or salicylic acid, and if both are there, put them on different nights. Retinoids are known to loosen the outer layer of skin. Splitting nights is a common approach; it hasn't been tested in a trial. The next morning, after cleansing, notice whether the sting has eased. If stinging and redness don't settle, or the skin swells or weeps, stop both and see a dermatologist.
• Drop the serum that keeps stinging right after you apply it, and wear sunscreen during the day. Irritation itself can leave or deepen brown marks after inflammation (post-inflammatory hyperpigmentation), and sun protection is a basic part of managing those marks (Davis & Callender, 2010). If the sting is gone the day after you drop a serum, that serum was the cause. If redness and brown marks keep darkening even after you drop it, see a dermatologist.