Can you mix tretinoin and benzoyl peroxide?

You might know that usually you shouldn’t mix tretinoin (all-trans retinoic acid) and benzoyl peroxide. Benzoyl peroxide is an effective acne treatment and works against acne-causing bacteria by oxidizing them.

A major benefit of benzoyl peroxide is that it is unlikely to cause bacterial resistance. It may even reduce the risk of bacterial resistance to topical antibiotics when used alongside them¹. However, the oxidizing action of benzoyl peroxide is indiscriminate and can cause tretinoin to breakdown – likely along its tail where there are many sensitive carbon double bonds.

Why might someone want to use tretinoin and benzoyl peroxide together? The combination is sometimes suggested by dermatologists and is effective² – if irritation is kept in check.

In a clinical trial³, the combination was shown to reduce acne faster and more effectively than each one alone. The two ingredients have some different effects on the skin, which may work together to give better results.

Experiments have found that mixing benzoyl peroxide with tretinoin can quickly lead to degradation – especially when they are exposed to light, with and without UV. While conditions were different in each experiment, within 24 hours of mixing and light exposure more than 80% of the tretinoin had degraded. In one experiment, 50% of the tretinoin had degraded within 2 hours.

Because of this information, it is often recommended to use one in the morning and the other in the evening.

In one of these experiments, adapalene was found to be stable against benzoyl peroxide’s oxidizing action and is often used as a substitute for tretinoin.

While this experimental evidence is convincing, other experiments suggest stability. Two experiments with tretinoin encapsulated in microspheres (Retin-A Micro), found that about 95% of the tretinoin remained after 8 hours of mixing with benzoyl peroxide and exposure to non-UV light. When exposed to UV light, about 80% of the tretinoin remained after 6 hours. An experiment with tretinoin that wasn’t encapsulated (Atralin Gel) also showed stability when mixed with 5% benzoyl peroxide. After 7 hours storage in an amber glass vial at 32ºC there was no degradation of the tretinoin.

Retin-A Micro’s encapsulation of tretinoin inside cross-polymer microspheres seems to have reduced the breakdown of it by benzoyl peroxide. This is most likely by physically separating the ingredients and reducing their ability to interact.

The second formula didn’t use encapsulation, but also had minimal-to-no degradation of the tretinoin. What stands out to me about the Atralin Gel is the butylated hydroxytoluene (BHT), an antioxidant commonly used in pharmaceuticals and food. Many other formulas of tretinoin have BHT, but it’s difficult to say whether they will also be stable when mixed with benzoyl peroxide. Only the Atralin Gel formula was tested.

These experiments show that the formulation of tretinoin largely determines how stable it is when mixed with benzoyl peroxide and exposed to light. There are differences between formulas available on the market, so a flat-out rule of “never mix” is likely false. This seems to be backed up by clinical reports of the combination of tretinoin and benzoyl peroxide being effective – as well as the experiments showing stability.

Twyneo is a new prescription that conveniently combines tretinoin encapsulated in silica core shells and benzoyl peroxide in the same formula. It has been shown to be stable. Adapalene is an alternative retinoid that is stable against benzoyl peroxide. Differin and other brands of adapalene are available over the counter in the United States.

For people without access to these formulas, those with encapsulated tretinoin like Retin-A Micro and an antioxidant like in Atralin Gel may help put the combination of tretinoin and benzoyl peroxide back on the table.

Talk to your dermatologist or doctor!


  1. The Role of Benzoyl Peroxide in the New Treatment Paradigm for Acne
  2. Retinoic acid cream (Airol cream) and benzoyl-peroxide in the treatment of acne vulgaris
  3. Case-based experience with the simultaneous use of a fixed topical antibiotic/benzoyl peroxide combination and a topical retinoid in the optimization of acne management
  4. The stability of tretinoin in tretinoin gel microsphere 0.1%
  5. Chemical stability of adapalene and tretinoin when combined with benzoyl peroxide in presence and in absence of visible light and ultraviolet radiation
  6. The effect of simulated solar UV irradiation on tretinoin in tretinoin gel microsphere 0.1% and tretinoin gel 0.025%
  7. Absence of Degradation of Tretinoin When Benzoyl Peroxide is Combined with an Optimized Formulation of Tretinoin Gel (0.05%)
  8. Twyneo® (Microencapsulated Benzoyl Peroxide 3%, Tretinoin 0.1%) Phase 3 Efficacy and Safety: Results From Two Randomized Controlled Clinical Trial

Does your skin need blue light protection from your devices?

From your phone or screens? Probably not.

When examining the results of studies looking at the effect of visible light, like blue light, we need to be really focused on the context.

Studies that have shown a decrease in collagen, an increase in free radical production, or an increase in cell death…have been done on human skin cells in a petri dish.

Those results will probably not translate to our skin.

Our skin has more layers, including the epidermis. The epidermis contains a distribution of melanin. Melanin absorbs visible light like blue, green, and red light.

Almost none of the effects observed in human skin cells in a petri dish have been found in human skin.

The big exception is hyperpigmentation, which has been observed in people with deeper skin tones (Fitzpatrick phototype 3 and greater).

But there’s a big and bright caveat…

That caveat is irradiance. There’s a difference between a dimly lit light and a brightly lit one, but somehow many articles on the topic forget about this. I don’t think it’s much of a stretch to say that the sun is brighter than your phone.

One experiment that showed hyperpigmentation in people with deeper skin tones used a blue light dose of 99 joules per square centimeter of skin. That’s estimated to be about one-and-a-half to two-and-a-half hours of direct sunlight in the summer.

But the light from a screen? You’d need about 2000 hours to get that same exposure and that’s assuming you’re holding the screen close to your skin.

The brightest TV screens at a distance of 30 cm (about 12 inches) from your face are delivering about 1/200th (0.5%) of the blue light from the sun.

Irradiance also follows the inverse-square law, so doubling the distance from that bright TV screen to 60 cm means you’re getting 1/4th of the energy or about 1/800th (0.125%) of the blue light from the sun.

Unlike UV light, there’s also no standard for blue light protection like SPF. It’s difficult to compare products, ingredients, methods of protection, and their effectiveness.

Many of us are already using sunscreens, pigments, and antioxidants which may help.

For those with deeper skin tones who are concerned about hyperpigmentation, remember irradiance. Be sun-safe when you’re out on a bright summer day.

Watching a video or gaming?

You can just chill.

Sources and further reading:

pH isn’t a scale between 0 and 14: Many diagrams are just incomplete!

pH (historically, power of hydrogen or potential of hydrogen) is often described as a scale that goes from 0 to 14. If we look at the formula that defines pH, we’ll see that it has no upper or lower bound.

pH = -log₁₀[H₃O⁺ (aq)]

pH is equal to the negative logarithm (base 10) of the concentration of hydronium ions (H₃O⁺) in water.

Sometimes H3O⁺ is written as H⁺, in the case with pH – it’s the same thing.

H⁺ is called a hydron, a proton, a hydrogen cation, or a hydrogen ion.

In the case of pH, since we’re looking at aqueous solutions, we assume the H⁺ is bound to a water molecule (H₂O).

H₂O plus a H⁺ is H₃O⁺.

If we look at this part of the formula: [H₃O⁺ (aq)]

The (aq) tells us it is aqueous – it is dissolved in water.

The square brackets tells us it is a concentration – a type called molarity or molar concentration. Molarity or molar concentration is the number of moles of solute (the thing dissolved) per liter.

Moles can be confusing, but one way to think of it is like a dozen.

A dozen = 12

A mole = 6.022×10²³ (6.022 times 10 to the power of 23)

You can have a dozen of something or a mole of something. A dozen hydronium ions is 12. A mole of hydronium ions is 602200000000000000000000.

Let’s say we have a solution of sugar in water with a molarity of 1.

That means if we measured out 1 liter of that sugar solution, it would contain 1 mole (6.022×10²³) of sugar molecules.

What is the pH of a solution with a 1.0×10⁻¹² (1 times 10 to the negative power of 12) hydronium ion molarity?
Or, in other words, what is the pH of a solution with 0.000000000001 moles of hydronium ions per liter?

pH = -log₁₀[10⁻¹²] = (-1)(-12) = 12

What is the pH of a solution with a 10⁻³ hydronium ion molarity?

pH = -log₁₀[10⁻³] = (-1)(-3) = 3

What about a hydronium ion molarity of 10⁻¹⁵?

pH = -log₁₀[10⁻¹⁵] = (-1)(-15) = 15

That’s above 14!

What about a hydronium ion molarity of just 10?

pH = -log₁₀[10] = (-1)(1) = -1That’s below 0!

We’re not likely to bump into things with a pH higher than 14 or below 0, that’s why diagrams often end there. But they do exist!

A 37% concentration of hydrochloric acid has a pH that’s around -1.

A saturated solution of sodium hydroxide has a pH of about 15.In California at the Richmond Mine of the Iron Mountain the pH of the water has been measured to be as low as -3.6!

While this may seem pedantic, I think it’s important to understand what we’re discussing and educating others about – especially in the beauty science community.

For example, did you know the way acid and bases are often described in the beauty community is only one of the theories?

It’s called the Arrhenius theory of acid and bases, but there’s also the Brønsted-Lowry theory and the Lewis theory!

Making sense of sunscreen protection percents. Where does “SPF 30 absorbs 97% of the UV” come from?

You may have heard that: “SPF 30 absorbs 97% of the UV” or “SPF 50 absorbs 98% of the UV”. These numbers are from a math model and it’s quite simple!

The math model is:

1 – (1 ÷ SPF)

1 divided by the sunscreen’s SPF, subtracted from 1.

With an SPF 30:

1 – (1 ÷ 30) = 1 – (1/30 or 0.0333…) = 1 – 0.0333… = 0.9666…

The ellipses (…) means repeating, the 666 in the decimal number 0.9666 repeats forever.

For simplicity, we can round up 0.9666… to 0.97. We can then convert a decimal number to a percent by multiplying it by 100.

0.97 x 100 = 97%

What’s the basis of this math model? The SPF of our sunscreens are tested experimentally on real people. SPF is the ratio between the amount of UV the participants’ skin can be exposed to before sunburn with and without the sunscreen.

SPF can be affected by things that aren’t absorbing or reflecting UV – like antioxidants, anti-inflammatories, protection boosters, and an individual’s skin. We also know that not every wavelength of UV causes sunburn equally. The math model only accounts for the amount of UV the sunscreen passes through to the skin and the amount of UV it doesn’t.

That’s why these percentage protection numbers are a model, they’re a simplified representation. But models can be useful in understanding complicated things.

So let’s break down this model

1 – (1 ÷ SPF)

1 ÷ SPF represents the fraction of UV that the sunscreen lets through.

So in the model, an SPF 25 exposes the skin to 1 ÷ 25 or 1/25 or 0.04

To convert a decimal number into a percent we multiply by 100
0.04 x 100 gives us 4%.

If we want to know the fraction of UV that the sunscreen prevents from reaching the skin in this model, we subtract it from the total, which is 100%. 100% can be written as 1/1 or 1 or 25/25.

1 – (1 ÷ SPF)

With an SPF 25, we can write 1 as 25/25

1 – (1 ÷ 25) = 1 – (1/25) = 25/25 – (1/25) = 24/25 or 0.96

To convert a decimal number into a percent we multiply 0.96 by 100, which gives us 96%.

The model doesn’t account for how, or really what form of UV. Just the UV that causes sunburn – which SPF is a ratio of, and what is being allowed through and not let through.

1 ÷ SPF gives us the fraction of UV the sunscreen lets through.

1 – (1 ÷ SPF) gives us the fraction of UV that the sunscreen doesn’t let through.

The fraction of UV that is being let through and not being let through add up to all of the UV, 1 or 100%.

On the previous slides, we showed that an SPF 25 in the model lets through 4% and doesn’t let through 96% of the UV.

4% and 96% add up to 100%.

Let’s run through this for an SPF 60. Working it out with your calculator can make it easier to understand!

1 ÷ SPF gives us the fraction of UV the sunscreen lets through.

1 – (1 ÷ SPF) gives us the fraction of UV that the sunscreen doesn’t let through.

Since the SPF is 60, we can put that in

1 ÷ SPF gives us the fraction of UV the sunscreen lets through. We can write 1 ÷ 60

1 – (1 ÷ SPF) gives us the fraction of UV that the sunscreen doesn’t let through.

We can write 1 – (1 ÷ 60)

What fraction or percent of the UV does this model show an SPF 60 letting through and not letting through?

So the amount of UV that an SPF 60 lets through in this model is:

1 ÷ SPF, since SPF is 60, we write 1 ÷ 60

1 ÷ 60 can be written as 1/60. Enter that into a calculator and you get the decimal number, which is 0.01666… for simplicity, we can round that up to 0.0167. We multiply that by 100 to get a percent, 1.67%

The amount of UV that the SPF doesn’t let through is 1 – (1 ÷ SPF). We know 1 ÷ SPF is 1.67%, so 100% minus 1.67% gives us 98.33%

1 – (1 ÷ SPF) = 1 – (1/60) = 60/60 – (1/60) = 59/60 = 0.98333… = 98.333% rounded to 98.33%

We can check our work by seeing that 1.67% and 98.33% add up to 100%.

Sometimes the percentages don’t add up to exactly 100% – that’s usually because of how the decimal numbers were rounded.

The math here might look complicated, but it is just fractions.

If you know a quarter is 1/4 and can be written as 0.25 or 25%

That 4 quarters is equal to 1 and can be written as 4/4 or 100%

Then you can do this!

Shame, Ageism, and Sunscreen

Many of us just didn’t grow up with good sun protection education.

I think a lot of us have forgotten that many of the bad effects caused by sun and UV exposure have only been recently well understood. While we’ve observed for a long time that sun exposure causes sunburn, the impact UVA has on skin’s appearance and photoageing are a relatively recent understanding and concern. Sunscreens marketed as an appearance maintaining essential are arguably modern.

The first widely used “sunscreen” was Red Vet Pet. Used by American soldiers during WW II, it was a by-product of oil refining with a strong red hue. In the later 1940s, pharmacist Benjamin Green would base his Coppertone product on it, but it was marketed to improve one’s ability to tan.

One of the first effective commercial sunscreens, Gletscher Crème, was introduced by Franz Greiter in 1946. Rudolf Schulze published the first method to measure sun protection in 1956. It’s estimated that Gletscher Crème only had a Schulze Factor of 2.

It wasn’t until 1974 that Schulze’s method would be adapted as the Sun Protection Factor and slowly start spreading around the world. In 1965, doctors J. Graham Smith and G. Rolland Finlayson presented their summary of the sun’s impact on skin, “The changes in human Caucasian skin commonly believed to be due to aging are primarily the effects of prolonged repeated damage to the skin from the sun”. There’s no discussion on the different effects caused by UVA and UVB.

One of the first standards to measure the UVA protection of a sunscreen was published in 1994 by Brian Diffey. It wasn’t until 2011 that the US FDA harmonized and set down rules as to what sunscreens could be labelled as “Broad Spectrum”.

While sunscreen use might reduce the risk of some skin cancers, it doesn’t reduce the risk of all of them. “Wear sunscreen to prevent skin cancer” messaging can be blunt and not inclusive. Dr. Adewole Adamson, a dermatologist, researcher and professor explains:

“In Blacks, melanoma usually develops in parts of the body that get less sun exposure, such as the palms of the hands and soles of the feet. These cancers are called ‘acral lentiginous melanomas,’ and sunscreen will do nothing to reduce the risk of these cancers…even among Whites, there is no relationship between sun exposure and the risk of acral lentiginous melanomas. Famously, Jamaican singer Bob Marley died of such a melanoma on his great toe, but sunscreen would not have helped.”

Sometimes we forget what it feels like to not know something – once we’ve learned it. A lot of the understanding of the sun’s effects and sunscreen protection labels are relatively modern. Not all of us had the opportunity to grow up in households or communities that were sun protection prescient. Not all of us knew the effects that prolonged sun exposure could have on our skin. Not all of us cared when we were younger.

To shame someone for not having consistently worn sunscreen throughout their life is to say that their skin – the interface of their body to the world – is irredeemable.

Would I prefer people to wear sunscreen more often? Yes, but you haven’t failed if you didn’t start wearing sunscreen when you were a child. Some people just don’t care about getting wrinkles or pigmentation. I think there needs to be space in the beauty community for them as well.

Sunscreen dosing. Teaspoons, shotglasses, and fingers…are we using too much?

Sunscreens and moisturizers with SPF are tested at a standardized density. That density is 2 milligrams of sunscreen per square centimeter of skin. If we want protection closer to what’s on the label, we should be using sunscreen or moisturizers with SPF at the density they’re tested at too.

Most of us don’t know the surface area of our skin and most of us don’t know the density of our sunscreens either. This has led to techniques and recommendations, like using 2 or 3 finger lengths of sunscreen, using 1/4 teaspoon of sunscreen, or applying our sunscreen twice. These techniques are all meant to encourage a more generous application of sunscreen, because when unprompted people tend to not apply enough.

With sunscreen, I think it is better to err on the side of applying too much rather than not enough. A higher density and thicker film of sunscreen generally means higher protection. It also makes sense to use more, because some of the sunscreen we apply will remain on our fingers, palms, or tools.

In some cases, I think these techniques might be leading people to use much more sunscreen than they might need. A sunscreen that might have been acceptable at a lower density might leave a strong cast, be too greasy, or pill (when a formula adheres to itself and rubs off the skin). This is especially relevant for people with deeper skin tones evaluating sunscreens for cast.

I’ve made a rough estimate of the surface area of my face, as well as the density (grams per millilitre) of two different sunscreens (IGTV: ‘How much sunscreen do you need?’). One is a cream and the other is a free-flowing milky texture. For both sunscreens, I need about 0.8 millilitres (mL) of sunscreen to get about the 2 milligrams of sunscreen per square centimeter of skin for my face. If I used a 1/4 teaspoon (1.23 mL), I’d be dispensing about half more than I need to protect my face.

With the cream sunscreen, applying 3 finger lengths of sunscreen dispensed about 3 mL of sunscreen. That’s almost 4 times more sunscreen than I might need. With the milk sunscreen, applying 3 finger lengths of sunscreen dispensed about 1.5 mL of sunscreen. That’s about 2 times more sunscreen than I might need. There’s also going to be differences in the thickness of our ‘lines’. That depends on things like how hard we squeeze, how slow we dispense, and the packaging.

If I apply the milky sunscreen with my palms and fingers the amount that ends up on my face might be close to the density I need. With 3 fingerlengths of the cream sunscreen, the finish is greasy and leaves a strong cast. The sunscreen is unusable for me with this much. But If I apply around 0.8 mL of the cream sunscreen, the finish is much nicer and I don’t notice a cast.

There’s no right or wrong method, they’re all just recommendations to encourage “proper” sunscreen use. Measuring your skin’s surface area, sunscreen density, and then each dose is going to be impractical for most people. But we might think about taking another look at sunscreens that may have been overused and left an unusable finish. They might not have at a density closer to the one it was tested at.

A photo showing 3 fingerlengths of a liquid, runny and milk appearing sunscreen. A photo showing those 3 fingerlengths of the milk appearing sunscreen collected in a syringe. It amounts to about 1.5 mL of sunscreen.

A photo showing 3 fingerlengths of a cream sunscreen. A photo showing those 3 fingerlengths of the cream sunscreen collected in a syringe. It amounts to about 3 mL of sunscreen.

One of the ways we can encourage use of sunscreen is modeling realistic use. Many people can’t stand wearing heavier coats of sunscreen, and that’s OK. Some sun protection is better that none.

On the opposite end, brands need to take responsibility for their marketing, and show actual and proper use – especially since they’re the ones testing it at the right density. A smidge might look nice in an advertisment, but we all know it’s not enough and misleading. In the US, sunscreens are drugs not cosmetics – brands need to respect that and stop playing around.

Cheers to @FiddySnails for popularizing the 2 and 3 finger methods. They work for her because her 3 finger-lengths are not as chonky as mine. As well, when using 3 finger-lengths she uses a cushion puff to apply – which is absorbing some of the sunscreen.

If they made 1/6th teaspoons, I’d be golden. You don’t need to overthink this like I did! Our first application of sunscreen doesn’t need to be “perfect” if we reapply throughout a very UV-exposed day. All these techniques, besides precise measurements, are just estimates or rules of thumb – so some common sense and adjustments are sometimes needed!