Serum 16 Vs Serum 24: Which Is Right For You?
Serum 16 and Serum 24 are two steps on the same route. Here's how to tell which one fits your skin, and when to move on.
Serum 16 and Serum 24 aren't rivals. They share the same vitamin A complex at two strengths, so the right one is simply the one your skin is ready for right now.
Serum 24 Is Back
While stocks last.
"Should I be on Serum 16 or Serum 24?" is one of the questions we're asked most. It's a fair one: the names are similar, the bottles look alike, and both promise smoother, brighter, firmer-looking skin.
The honest answer is that it depends less on your skin concern and more on how well your skin already handles vitamin A. Choose too gently and progress stalls. Jump too high too soon and you'll spend weeks managing flaking and redness instead of seeing results.
Here's how the two serums relate, where Define fits, and the signs that tell you it's time to step up.
Two steps on one route
Both serums are built on CosMedix's LG-Retinex complex: a blend of retinol, retinaldehyde and glycosaminoglycans. Retinol and retinaldehyde do the renewing; the glycosaminoglycans help the skin hold on to water while they work.
The difference is concentration. Serum 16 contains the complex at 16%; Serum 24, from the CosMedix Elite range, contains it at 24%. Same delivery system, same texture, same routine. One simply asks more of your skin than the other.
Build the foundation
A calm, well-hydrated barrier and daily SPF. Vitamin A works best on skin that isn't already compromised.
Serum 16 — the professional-strength starting point
16% LG-Retinex complex. Introduced every one to three nights, then built up to nightly as tolerated.
Serum 24 — the next step
24% LG-Retinex complex. For skin that's settled into nightly vitamin A and is ready for more.
What 'comfortable with vitamin A' means
You'll often hear that Serum 24 is for people who are "comfortable with vitamin A". That doesn't mean you've used a retinol once. It means your skin has genuinely adapted. Look for all four of these:
Nightly, not occasionally
You've been using your current vitamin A most nights for several weeks, not just two or three times a week.
Flaking has settled
Any early dryness or peeling has passed, and your skin no longer feels tight after cleansing.
No lingering sensitivity
Products that don't normally sting don't sting now. No persistent redness or heat.
SPF is a habit
You wear broad-spectrum SPF every morning, whatever the weather. That's non-negotiable on any vitamin A.
If you can tick all four on Serum 16 and your results have started to plateau, that's the moment to think about Serum 24.
Serum 16 vs Serum 24 vs Define
Define often comes up in the same conversation. It's a different kind of product: a cream that pairs 2% encapsulated retinol with a blend of AHAs (lactic, malic and tartaric acid), so it renews and resurfaces in one step.
| Define | Serum 16 | Serum 24 | |
|---|---|---|---|
| Range | CosMedix | CosMedix | CosMedix Elite |
| Vitamin A | 2% encapsulated retinol | 16% LG-Retinex complex (retinol + retinaldehyde) | 24% LG-Retinex complex (retinol + retinaldehyde) |
| Also contains | Triple AHA blend | Glycosaminoglycans, hyaluronic acid, squalane | Glycosaminoglycans, hyaluronic acid, squalane |
| Texture | Cream | Serum | Serum |
| Best for | Texture and dullness, where resurfacing is the priority | Building a consistent vitamin A habit; lines, tone and texture | Established vitamin A users whose progress has plateaued |
| How to use | Evenings, after cleansing | 1 pump, every 1–3 nights, building to nightly | 1 pump, every 1–3 nights, building to nightly |
Because Define already contains acids, don't layer it with Serum 16 or 24 on the same night unless your practitioner has advised it.
Introducing the next step
Serum 24 is the most concentrated version of the LG-Retinex complex in the range. CosMedix reports that in its eight-week study, 91% of participants saw fewer visible fine lines. Results like that depend on consistency, though, and consistency depends on not overwhelming your skin in week one.
Moving from Serum 16 to Serum 24 is a step up, not a fresh start. Treat it like one:
Weeks 1–2: alternate
Use Serum 24 every third night and Serum 16 on the others, so your skin meets the higher strength gradually.
Weeks 3–4: every other night
If skin stays calm, increase Serum 24 to alternate nights. Follow with moisturiser if you need extra hydration.
From week 5: nightly as tolerated
Move to nightly Serum 24 only if there's no lasting redness, stinging or flaking. If there is, hold at the previous frequency for longer.
A simple vitamin A routine
AM
- Cleanse with Purity Clean. If your skin feels sensitive after a step-up, keep it to the morning only.
- Moisturise if needed.
- Finish with SPF50, such as Peptide Rich Defence. Always the last step, every day.
PM
- Cleanse gently and pat skin dry.
- One pump of Serum 16 or Serum 24, at your current frequency.
- Moisturiser if your skin needs extra hydration.
Once or twice a week
- Pure Enzymes Exfoliating Mask on a night you're not using vitamin A, never on top of it.
- Skip it entirely while you're stepping up strength or if skin feels reactive.
If your skin reacts
Some response is normal when you increase vitamin A strength. The key is telling purging (temporary, and a sign of faster cell turnover) apart from irritation (a sign your barrier needs support).
Likely purging — continue
- Small breakouts in areas where you usually break out
- Spots that come and go faster than usual
- Mild, short-lived dryness
- Settles within roughly four to six weeks
Likely irritation — step back
- Redness, burning or stinging that lingers
- Breakouts or rash in new areas
- Raw, tight or shiny-looking skin
- Gets worse rather than better
If it's irritation, pause vitamin A for a few nights and focus on barrier repair: gentle cleansing, moisturiser and SPF. When skin is calm again, restart at a lower frequency or go back to Serum 16 for a few more weeks. There's no prize for rushing.
Try the sandwich method: a thin layer of moisturiser, then your serum, then moisturiser again. It buffers the vitamin A without stopping it working.
Serum 16 and Serum 24 are facial treatments. Keep them away from the eyelids and lash line; experienced users may apply cautiously around the outer crow's feet only when directed by a practitioner.
Pregnancy and breastfeeding
Retinol and retinaldehyde are forms of vitamin A, and topical vitamin A is generally avoided while trying to conceive, during pregnancy and while breastfeeding. That applies to Serum 16, Serum 24 and Define.
If any of these apply to you, pause your vitamin A and check with your GP or midwife before using any active skincare. We're always happy to suggest a vitamin A-free routine to keep your skin on track in the meantime.
Ask us
If you can't tell whether you're ready for Serum 24, or your skin is doing something you don't recognise, get in touch. Tell us what you're using now, how often, and how your skin feels, and we'll point you to the right step.
The rule of thumb Start where your skin is comfortable. Step up when it's calm, consistent and plateauing, not before.Featured in this guide
The highest-strength LG-Retinex complex in the range: 24% retinol, retinaldehyde and glycosaminoglycans to target lines, discolouration, texture and tone. Comes with a free Pure Enzymes Exfoliating Mask 60ml.
Professional-strength 16% LG-Retinex complex for building a consistent vitamin A habit, targeting fine lines, age spots and uneven texture.
A cream combining 2% encapsulated retinol with a triple AHA blend to exfoliate and support collagen and elastin. Use in the evening.
Cranberry enzymes and L-lactic acid gently loosen surface build-up and clear pores, leaving a soft, dewy finish. Once or twice a week, on non-vitamin A nights.
A concentrated cleanser with 5% L-lactic acid and cooling peppermint to unclog pores. Suits normal to oily skin; keep to mornings while stepping up vitamin A.
A lightweight, hydrating broad-spectrum SPF50 with peptides, squalane and vitamin E. The final step of every vitamin A morning.
Ready for the next step?
If your skin is calm, consistent and ready for more, Serum 24 is back in stock, and it comes with a gift.
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