Unit 2 · Vitamin A — the retinoids
Why "retinoid" is not one ingredient but a five-step potency ladder
Every retinoid on the shelf is judged against the same active molecule, tretinoin, and every form other than tretinoin has to be converted to it before it can do anything. By the end of this unit you should be able to explain why that conversion chain determines potency, and when a gentler form is the clinically correct choice rather than a compromise.
Learn · Mechanism of action
Nuclear receptor binding and the downstream effects it triggers
Vitamin A and its derivatives — collectively called retinoids — are the most comprehensively studied topical actives in aesthetic medicine. Tretinoin (all-trans retinoic acid) remains one of the few topical ingredients with Level I evidence for reversal of photoageing.
Retinoids exert their effects primarily by binding to nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs) — transcription factors that regulate the expression of hundreds of genes.
- Collagen synthesis
- Stimulation of procollagen I and III synthesis.
- Collagen breakdown
- Suppression of matrix metalloproteinases (MMPs) that break down collagen.
- Epidermal turnover
- Normalisation of epidermal turnover — cell cycle acceleration.
- Exfoliation
- Reduction of keratinocyte cohesion in the stratum corneum.
- Pigmentation
- Inhibition of melanogenesis.
Only tretinoin binds directly to RARs. So what has to happen before retinaldehyde, retinol or a retinyl ester can activate any of the gene expression described above?
Hold your answer before you open this. The value is in having committed to a mechanism first.
All other retinoid forms — retinaldehyde, retinol, retinyl esters — must be enzymatically converted by the skin to retinoic acid before they can activate gene expression. Each conversion step introduces inefficiency, which is what produces the potency gradient across the retinoid family.
The retinoid irritation response — dryness, peeling, erythema — is also mediated through RAR signalling. It is not an allergy but a pharmacological effect of retinoic acid activity, and it generally diminishes with continued use as the skin acclimatises. That distinction is worth stating plainly to a patient who is deciding whether to persevere through the first fortnight.
A patient asks why her new prescription-only cream seems to work faster than the over-the-counter retinol serum she used previously. Which retinoid form requires no enzymatic conversion to reach its active form?
Select an option to commit. The reasoning appears afterwards.
Tretinoin is retinoic acid itself — it requires no enzymatic conversion and binds RARs directly. Every other form in the retinoid family — retinaldehyde, retinol, retinyl esters — must be converted to retinoic acid by the skin before it can activate the same gene expression pathway, and each conversion step reduces effective potency.
This is the mechanistic reason a prescription retinoid typically produces visible change faster than an OTC retinol at an equivalent nominal strength: there is no conversion bottleneck standing between application and receptor activation.
Learn · The retinoid conversion ladder
From tretinoin to bakuchiol — potency, evidence and scheduling
The retinoid family spans a wide potency range, driven almost entirely by how many conversion steps sit between the applied product and retinoic acid at the receptor.
| Form | Conversion steps | Relative potency | Notes |
|---|---|---|---|
| Tretinoin (retinoic acid) | None — active form | Very high | Prescription only (AU). Gold standard for photoageing and acne. Significant irritation potential. |
| Adapalene | None — synthetic retinoid | High | Selective RAR-β/γ agonist. Less irritating than tretinoin. TGA-approved OTC (0.1%) for acne. Excellent anti-comedogenic. |
| Retinaldehyde (retinal) | 1 step | Moderate–high | Strongest available OTC retinoid in most markets. Also has direct antibacterial effect. Common in professional-grade retail (0.05–0.1%). |
| Retinol | 2 steps | Moderate | Most common OTC retinoid. Wide range of concentrations (0.025–1%+). Effective with consistent use; results take longer than tretinoin. |
| Retinyl esters (palmitate, acetate, propionate) | 3+ steps | Low | Gentlest option. Suitable for very sensitive skin or retinoid-naive patients. Often a marketing inclusion rather than an active dose in mass-market products. |
| Bakuchiol | Not a retinoid — plant-derived | Low–moderate | Functional retinoid alternative. Activates some RAR pathways. Significantly gentler. Emerging evidence for comparable anti-ageing effects at 0.5–1%. Covered in full in Unit 5. |
Tretinoin (all-trans retinoic acid) is Prescription Only (Schedule 4) in Australia. It requires an authorised prescriber and is not available over the counter. Adapalene is TGA-approved for over-the-counter supply at 0.1% for acne. All other retinoid forms above are available without prescription at the concentrations listed.
Learn · Matching retinoids to presentation
Photoageing, acne, pigmentation, sensitivity and pregnancy
The strongest evidence base sits with photoageing and acne. Everything else on this list is a modifier on how carefully — not whether — a retinoid is used.
- Photoageing & fine lines
- First-line topical, with the strongest evidence base of any active in this module. Start with retinol 0.025–0.05% and increase slowly; tretinoin 0.025–0.1% is for patients under prescriber supervision.
- Sensitive or rosacea-prone skin
- Retinyl esters or bakuchiol are preferred. True rosacea with active erythema may worsen initially with a retinoid — introduce only after the barrier is stabilised.
Acne & comedonal congestion: retinoids normalise follicular keratinisation, addressing the primary pathophysiological step in comedone formation. Adapalene is preferred for acne — less irritating, strong evidence, OTC available. Tretinoin is effective for both inflammatory and non-inflammatory acne.
Post-inflammatory hyperpigmentation (PIH): retinoids accelerate pigmented cell turnover and inhibit tyrosinase activity, which is supportive for PIH. However, retinoid dermatitis — barrier disruption, erythema — can trigger new PIH in darker phototypes. Introduce very slowly and combine with barrier support such as panthenol or niacinamide.
In Fitzpatrick IV–VI, the risk is not the retinoid itself — it is the irritation response triggering PIH. Start at the lowest concentration, use every second to third night, and ensure concurrent SPF and barrier support.
All retinoids are contraindicated in pregnancy. Bakuchiol is the recommended alternative.
A patient with Fitzpatrick V skin, with no prior retinoid use, wants to start a retinoid for early photoageing. The safest initiation approach is:
Select an option to commit. The reasoning appears afterwards.
In Fitzpatrick IV–VI, the risk is not the retinoid itself — it is the irritation response triggering PIH. Retinoid dermatitis, meaning barrier disruption and erythema, can trigger new post-inflammatory hyperpigmentation in darker phototypes even when the underlying retinoid mechanism is working exactly as intended.
The correct sequence is the lowest available concentration, applied every second to third night rather than nightly, with concurrent SPF and barrier support such as panthenol or niacinamide. That sequence manages the irritation pathway that actually causes the complication, rather than avoiding a beneficial active out of caution about the wrong mechanism.
A patient in her first trimester of pregnancy asks about starting a retinoid to manage early photoageing. The appropriate response is:
Select an option to commit. The reasoning appears afterwards.
All retinoids are contraindicated in pregnancy, without a concentration- or trimester-specific exception. This applies across the entire potency ladder covered in this unit, from tretinoin down to the gentlest retinyl ester.
Bakuchiol is the recommended alternative — it is described in the source material as a functional retinoid that activates some of the same RAR pathways without the contraindication. Unit 5 covers its evidence base and appropriate use in full.
Learn · Clinical application
The initiation protocol that keeps patients on treatment
The most common reason patients discontinue retinoids is the initial irritation response. A staged initiation schedule manages that response without materially slowing outcomes.
| Timepoint | Frequency |
|---|---|
| Weeks 1–4 | Every second to third night |
| Weeks 5–8 | Every second night |
| Week 9+ | Nightly, if tolerated |
A "retinoid sandwich" — applying moisturiser before and after the retinoid — reduces barrier disruption without significantly compromising efficacy in retinoid-naive patients. Always instruct patients on strict daily broad-spectrum SPF while using retinoids: the epidermis is thinner and more UV-sensitive during active retinoid therapy.
Unit 2 summary
Clinical takeaways
- Only tretinoin binds RAR directly. Every other retinoid form needs enzymatic conversion first, and each conversion step is what produces the potency gradient across the family.
- Scheduling tracks potency but is not identical to it. Tretinoin is Prescription Only (Schedule 4) in Australia; adapalene is TGA-approved OTC at 0.1%; retinaldehyde, retinol, retinyl esters and bakuchiol are all available without prescription at their respective concentrations.
- In Fitzpatrick IV–VI, manage the irritation, not the retinoid. PIH risk comes from the barrier disruption a retinoid can provoke, not from the retinoid mechanism itself — low-and-slow initiation with barrier support addresses the actual risk.
- Pregnancy is an absolute contraindication. No retinoid form or concentration is exempt. Bakuchiol, covered fully in Unit 5, is the recommended alternative.