How to layer acids, retinal and vitamin C without upsetting your skin

If you already own a retinal, an acid and a vitamin C serum, the question is rarely what to buy next. It is what to use tonight, and what to leave in the cupboard.

Here is the short version. Vitamin C in the morning. Sunscreen every morning, without exception. Retinal on two or three nights. An acid on one or two other nights. Moisturiser every night, including the active nights. And while you are still building up, not an acid and a retinal on the same night.

That weekly shape is our own practical guess, and we would rather say so at the start. The studies below used daily application, not alternating schedules, so nothing here shows that three nights a week is the right number. What the studies do tell you is how skin behaved when it was pushed, and that is enough to plan around.

One more caveat worth having upfront. Several of these trials used tretinoin, which is a prescription medicine in New Zealand and is not what we sell. Retinal is a cosmetic ingredient. The tretinoin work is useful for understanding how vitamin A behaves on skin in general, but you cannot assume a cosmetic product does what a prescription cream did in a trial.

Stronger is not the goal

In a 48-week double-blind, vehicle-controlled trial with 99 people, tretinoin at 0.1% and at 0.025% improved the appearance of photoaged facial skin to a similar degree, with no clinically or statistically significant difference between the two strengths. Under the microscope the epidermis thickened by 30% and 28% respectively. What did differ was irritation. Redness and scaling were statistically significantly greater on the stronger one (Griffiths et al., 1995).

So if your skin is unhappy, moving up a strength is not the answer, and staying low is not a failure. The Crystal Retinal range also starts lower than most people realise. Crystal Retinal 1 is the entry point and Crystal Retinal 3 is the next step, and sitting on either of them for months is a sensible plan rather than a beginner's mistake.

The forms of vitamin A also behave differently from one another. In a patch test comparison, retinol and retinaldehyde had a low irritation potential while retinoic acid was clearly more irritating, on both clinical scoring and laser Doppler blood flow (Fluhr et al., 1999). That patch test involved six people, which is very small, so treat it as a signal rather than something to bank on. The comparison also came from particular formulas at particular percentages, and both the concentration and the base a product is built on change how it behaves.

The first month is the hard part

The same paper reported a longer arm: 355 people using retinaldehyde or retinoic acid over 44 weeks. With retinoic acid, 44% had redness, 35% had scaling and 29% had burning or itching, and those complaints came in the first four weeks of use. All three were significantly less frequent with retinaldehyde (Fluhr et al., 1999).

Irritation is front-loaded. That is the most useful thing to know when you are starting. Building up slowly is not being precious, it matches what was actually observed. One night a week for a fortnight, then two nights, then hold there for a while. Only add a second active once the first one has become boring. The two or three nights in the plan above is where you end up, not where you start.

Use moisturiser from the first night

In a paired, double-blinded study, women with facial photodamage applied one moisturiser to one side of the face and a different one to the other, twice daily for ten weeks. One contained niacinamide, panthenol and tocopheryl acetate. The other had similar moisturising ingredients without them. Tretinoin 0.025% went on the whole face from week two. The researchers reported that conditioning the skin first with the vitamin-containing moisturiser, and continuing it through treatment, eased the early weeks and gave a better response to the tretinoin (Draelos et al., 2006).

The useful part is the timing. Moisturiser goes on generously alongside your retinal from night one, rather than being brought in later as a rescue. Two limits worth knowing: the published abstract does not report how many women took part, and the study was built around a prescription cream. Read it as support for the habit, not as proof about any particular moisturiser.

Acids and sun, which matters more than the timing

This is the part that usually gets told back to front. Applying 10% glycolic acid at pH 3.5 once daily, six days a week for four weeks, left skin more sensitive to UV. Sunburn cell formation went up and it took less UV to redden the skin. A marker of UV damage to DNA was raised too, but not to a statistically significant level (Kaidbey et al., 2003). A separate study, applying 10% glycolic acid five days a week for three and a half weeks, found increased redness, DNA damage and sunburn cell formation, while salicylic acid 2% produced no significant change in any of those measures (Kornhauser et al., 2009).

Both studies were small and short, and both were done on back skin rather than faces. Twenty-nine people in the first, split into smaller groups of sixteen and thirteen for different measurements, and fourteen in the second. Everyone in the first study was Caucasian, so it does not describe every skin tone. Facial skin is not back skin either. Hold the numbers loosely.

Here is the bit that gets misread. In both studies the extra sensitivity was measured during a course of daily use, so applying your acid at night does not switch it off during the day. Evening use is still a reasonable habit and overnight acid products are formulated for that slot, but the thing that actually protects you is sunscreen every morning for as long as the acid is in your routine. With the UV index we get through a New Zealand summer, that is the most important line on this page. If an overnight acid suits you, Sleep Glycolic is a 10% free glycolic formula built for that slot, and the sunscreen rule applies to it like any other.

If you spend a lot of time outdoors and you are choosing between an AHA and a BHA, the salicylic acid result is a fair point of difference. It came from fourteen people in a single study. It is not a clean bill of health, and it is not permission to skip sunscreen on a BHA day.

There is something reassuring in there too. A week after the glycolic acid was stopped, the differences in sunburn cells and in the UV dose needed to redden skin had gone (Kaidbey et al., 2003). If you have been heavy-handed, a week off has something measured behind it.

Where vitamin C fits

The most quoted vitamin C study was done on pig skin in a laboratory, so start there. It measured how L-ascorbic acid gets into skin. It did not measure how anyone looked afterwards. In it, L-ascorbic acid had to be formulated below about pH 3.5 to enter the skin at all, 20% was the maximum useful concentration, tissue levels saturated after three daily applications, and the half-life of disappearance was about four days (Pinnell et al., 2001).

Two narrow things follow. There is no absorption argument for applying L-ascorbic acid twice a day, because levels saturated with once-daily use in that model. And the same study found that the derivatives it tested, magnesium ascorbyl phosphate, ascorbyl-6-palmitate and dehydroascorbic acid, did not raise skin levels of L-ascorbic acid at all. Plenty of vitamin C serums use a derivative rather than L-ascorbic acid, so those pH and concentration numbers do not automatically describe the bottle on your shelf. Worth reading the label rather than assuming.

Morning vitamin C and night retinal is a convenient split and nothing more. We could not find a trial showing the two interfere with each other, and we could not find one showing they do better apart. It is simply easy to remember. While we are on habits that get repeated as rules, we could not find a study testing how long to wait between layers either. Twenty minutes, sixty seconds, we have not found any of it tested. It is convention.

Signs to ease off, and how to come back

These are the things the studies actually measured (Fluhr et al., 1999), and in the tretinoin study the researchers described side effects of this kind as consistent with the skin's outer layer being compromised (Draelos et al., 2006):

• Redness that is still there in the morning, not just after applying
• Flaking, or rough patches that catch on makeup
• Tightness through the day
• Stinging from products that used to feel like nothing

If that is you, stop the actives for a week or two. Keep cleansing gentle, use a plain moisturiser generously, and keep the sunscreen going. Then restart one product only, at one night a week, and bring the second back once the first is comfortable. Resist restarting everything at once the moment your skin settles, because that is usually how people end up back here a fortnight later.

A few plain safety notes

• Sunscreen every morning while you are using any of these, and reapply it. Our UV index is high enough that this is the part to get right.
• Patch test anything new on a small area first and give it a few days.
• Do not apply actives to skin that is broken, weeping or already sore.
• If you are pregnant or breastfeeding, check with your doctor or pharmacist before using vitamin A products.
• If a doctor has prescribed something for your skin, ask them before adding anything alongside it. Several of the studies here involved prescription treatments, and that is a conversation for the person who prescribed yours.

What these products cannot do

These are cosmetic products. They work on the appearance of skin. Nothing in the evidence above supports using them to treat, heal or repair a skin condition, and redness or flaking from over-use is not a diagnosis. If skin is broken, weeping, painful, spreading, or simply not settling within a couple of weeks of stopping, that is a conversation for a doctor or pharmacist, not a serum.

An antioxidant serum is also not a substitute for sunscreen, and nothing here suggests otherwise.

One study gets quoted as a licence to stack, so it is worth heading off. A single co-formulated product containing 0.1% retinaldehyde and 6% glycolic acid was well tolerated over 90 days by 1,709 people using it alongside their usual acne treatments, and complaints were rare (Dréno et al., 2005). That was one product, formulated together at fixed low strengths, in people who were not using other retinoids with it, and no control group is described. It says nothing about layering two separate products you bought individually, and nothing about what our products do.

Sunscreen, and when to reapply

A modelling paper asked whether the usual advice to reapply every two to three hours is the best approach, and concluded it is not. Applying liberally 15 to 30 minutes before going out, then reapplying 15 to 30 minutes after sun exposure begins, gave lower UV exposure than waiting two hours. Reapplying at 20 minutes gave 60% to 85% of the exposure received when reapplying at two hours (Diffey, 2001). The model assumed a product that binds reasonably well to skin, of the kind sold as water resistant, and it found that a sunscreen which comes off easily achieves little whenever you reapply it. Reapply after swimming, towelling or heavy sweating regardless.

It is a mathematical model rather than a trial in people, so treat it as reasoning rather than measurement. The practical takeaway for a New Zealand summer is that applying enough, and reapplying early, does more for you than chasing a bigger number on the bottle. If your morning already ends with a serum, an SPF 50 day cream such as Advanced Day Ultimate Protect keeps it to one step, which is usually the difference between doing it and meaning to. If you are going to be outside for hours, a dedicated sunscreen applied generously will serve you better than any day cream.

If you are not sure how your particular set of products should sit across a week, bring them in to us in Parnell and we will map it out with you.