Face serums are the powerhouses of a skincare range concentrated, active-led, and exactly the products customers expect to deliver results. That concentration of actives is also what makes serums one of the more demanding products to get right from a safety point of view. This guide explains what a CPSR covers for face serums and the issues that most often need attention.
For the fundamentals, see our guide to what a CPSR is. Here we focus on serums and their actives.
What makes a serum different
A serum is typically a leave-on, concentrated product designed to deliver active ingredients to the skin vitamin C, niacinamide, hyaluronic acid, exfoliating acids, peptides and the like. It stays on the face, often twice daily, and is built around higher levels of actives than a general moisturiser. That combination potent ingredients, leave-on exposure, a sensitive facial site is precisely what the safety assessment has to weigh.
Because the whole point of a serum is its actives, the assessment focuses heavily on what those actives are, at what concentration, and whether that is safe and permitted. A serum is not inherently risky, but it is a product where ingredient choices have more consequence than in a simple, low-active formula.
Active ingredients and concentration limits
Many popular actives carry restrictions or recommended maximum levels. Some are restricted under the annexes with hard limits and conditions; others are governed by scientific opinions that set safe-use concentrations. Exfoliating acids, certain forms of vitamins, and various functional actives all have levels beyond which they are no longer considered safe for use, particularly leave-on.
This is the heart of a serum CPSR: checking each active against its limits and the latest guidance. Putting an active in at a high “clinical” level for marketing impact can tip it past what is permitted or safe, so the assessment confirms your levels are within bounds. Our guide to restricted ingredients explains how concentration limits work.
pH and acid-based serums
Acid serums those built around alpha-hydroxy acids (AHAs) like glycolic or lactic acid, or beta-hydroxy acids (BHAs) like salicylic acid depend on a low pH to work, and that same low pH is what makes them potentially irritating. Both the acid concentration and the pH determine how strong and how risky the product is, and these are constrained for cosmetic use, often with mandatory conditions and warnings.
The safety assessment looks at acid type, concentration and pH together, because they jointly determine the effect on the skin. Exfoliating-acid products commonly require specific label warnings for example about sun sensitivity and using sun protection which the assessor identifies. Getting the chemistry and the warnings right is essential for an acid serum.
Vitamin C and unstable actives
Some star actives are chemically unstable. Pure vitamin C (L-ascorbic acid) is the classic example: it oxidises readily, which is why vitamin C serums can turn yellow or brown and lose potency. Instability is both a performance and a safety matter, because a degrading active can change the product's pH, colour and behaviour over time.
This is where stability testing and sensible packaging (air-tight, light-protective) matter, and where the safety assessment's interest in how the product behaves over its shelf life comes in. A serum that is fine on day one but degrades within weeks needs either reformulating, stabilising, or an honest, short shelf life. Our stability testing guide covers the principles.
Preservation of water-based serums
Most serums contain water (or water-based ingredients like aloe or hydrosols), which means they can grow microbes and therefore need a preservative system and a challenge test. Lightweight, watery textures are no exception if anything, a thin aqueous serum is an excellent environment for microbial growth. “Preservative-free” is not a safe option for a water-based serum.
Anhydrous oil serums (facial oils with no water) are different they generally do not need a traditional preservative, relying instead on antioxidants to protect the oils. Knowing whether your serum is water-based or anhydrous is the first question in working out its preservation needs. Our preservatives and challenge test guides explain both cases.
Combining actives safely
Serums often combine several actives, and combinations bring their own considerations. Some actives interact affecting each other's stability, the product's pH, or the skin's tolerance when used together. A formula that stacks multiple potent actives can become more irritating than any one alone, and can be harder to keep stable and within limits across the board.
The safety assessment considers the formula as a whole, not just each ingredient in isolation, which is why the complete, accurate formulation is essential. A thoughtfully balanced combination can be both effective and safe; an everything-at-once approach is more likely to run into irritation, stability or compliance problems.
Claims and substantiation
Serums are sold on their claims brightening, smoothing, hydrating, reducing the appearance of fine lines and those claims must be truthful, supported and clearly cosmetic. Claims that stray into treating, curing or changing the structure of the skin can reclassify the product as a medicine. “Reduces the appearance of” is cosmetic language; “treats” or “cures” is not.
Where you make a specific performance claim, you should have evidence to support it, which may mean testing. Keeping claims within cosmetic bounds and backing them appropriately protects your compliance and your credibility. This is an area marketplaces and regulators watch closely for skincare actives.
Retinoids and other high-scrutiny actives
Some serum actives attract particular regulatory attention. Vitamin A derivatives (retinoids) such as retinol are effective but potent, and their use in cosmetics is subject to limits and conditions because of safety concerns about cumulative exposure. There has been ongoing regulatory tightening around retinol levels, so a retinol serum needs careful checking against the current permitted concentrations for its product type, and may require specific warnings.
Other actives in this high-scrutiny group include certain potent exfoliating acids, some preservative-boosting ingredients, and actives where the line between a cosmetic effect and a pharmacological one is thin. The common thread is that the more an active does, the more closely it is regulated. For a maker, the practical implication is to flag any “star” active early and confirm its current cosmetic limits before building a product and its marketing around it limits move, and a level that was acceptable a couple of years ago may have been reduced.
This is exactly the kind of moving target a safety assessor tracks. Rather than formulating to a number found in an old recipe or a competitor's label, it is far safer to confirm the current position for your specific active and product type as part of the CPSR. An active-led serum lives or dies on getting these levels right.
Packaging serums properly
Packaging is part of a serum's safety and stability story, not an afterthought. Many serum actives are sensitive to air and light, so airless pumps, opaque or tinted bottles, and well-sealed droppers help protect potency and keep the product stable over its shelf life. A vitamin C serum in a clear, wide-open jar will degrade far faster than the same formula in an airless, light-protective container.
Packaging choice also feeds into stability testing and compatibility: the product should be tested in the actual pack it will be sold in, since the container can interact with the formula. Choosing appropriate packaging from the start supports both a longer, more honest shelf life and a more robust safety assessment and spares you discovering a stability problem only after launch.
Key safety points for serums at a glance
| Consideration | Why it matters |
|---|---|
| Active concentrations | Many actives have limits or safe-use levels |
| pH (acid serums) | Drives both efficacy and irritation potential |
| Stability | Actives like vitamin C degrade and change the product |
| Preservation | Water-based serums need a preservative and challenge test |
| Active combinations | Interactions affect stability, pH and tolerance |
| Claims | Must be supported and clearly cosmetic, not medicinal |
Face serum checklist
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Identify every active and its concentration.
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Check actives against limits and safe-use levels.
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Control pH and add required warnings for acid serums.
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Plan for stability of unstable actives like vitamin C.
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Preserve and challenge-test water-based serums.
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Assess active combinations as a whole formula.
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Keep claims supported and cosmetic; assess it all in a CPSR.
Launching a face serum? Phoenix Safety Consultants assesses active-led serums properly concentration limits, pH, stability, preservation and claims and provides your CPSR, PIF and notification for the UK and EU.
Get a Serum CPSR →Frequently asked questions
Do face serums need a CPSR?
Yes. A serum is a cosmetic and needs a CPSR, PIF, Responsible Person, notification and compliant labelling. Its concentrated, leave-on actives make the assessment particularly important.
Are there limits on active ingredients in serums?
Often yes. Many actives are restricted under the annexes or governed by safe-use levels in scientific opinions. The CPSR checks each active's concentration against its limits, especially for leave-on use.
Why do acid serums need warnings?
Exfoliating acids work at low pH and can irritate and increase sun sensitivity. Their concentration and pH are constrained, and they commonly require specific label warnings, such as using sun protection, which the assessor identifies.
Do water-based serums need a preservative?
Yes. Any serum containing water can grow microbes and needs a preservative system and a challenge test. Anhydrous oil serums generally do not, relying on antioxidants instead.
Why does my vitamin C serum change colour?
Pure vitamin C oxidises easily, turning yellow or brown and losing potency. This is a stability and safety matter, addressed through formulation, protective packaging and realistic shelf life supported by stability testing.
Can I combine lots of actives in one serum?
You can, but combinations need care — actives can affect each other's stability and the product's pH, and stacking potent actives can increase irritation. The CPSR assesses the whole formula together.
What claims can I make for a serum?
Cosmetic claims like brightening or reducing the appearance of fine lines, provided they are truthful and supported. Avoid medicinal claims to treat or cure, which can reclassify the product as a medicine.
References: Regulation (EC) No 1223/2009 (EUR-Lex); Annex III restricted substances; SCCS opinions on cosmetic actives; ISO 11930; UK Cosmetics Regulation as retained. General information only, not legal advice.
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