A+ High-Dose Retinoid Serum

Let's start with the number that sells this serum: 6.5%. It's the biggest number on the marketing materials, and in a skincare market where consumers have learned to ask about concentrations, it sounds impressively potent. But the math behind that number deserves a closer look, because it tells a more nuanced story than the headline suggests.

The 6.5% breaks down into three components: 5% retinoid esters (gentle vitamin A derivatives that need multiple enzymatic conversion steps to become active), 1% liposome-encapsulated retinol blend (traditional retinol in a slow-release delivery system), and 0.5% blue-green algae extract with retinoid-like activity. The ingredient that actually matters most — Hydroxypinacolone Retinoate, the 'high-dose retinoid' in the product name — is present at approximately 0.5%. That's not a criticism of the concentration itself; 0.5% HPR is a meaningful amount given the ingredient's direct receptor-binding activity. But the 6.5% figure adds together ingredients of vastly different potencies and calls it a total, which is like adding the horsepower of a bicycle to a car and marketing the combined number.

Now, setting the marketing aside, the actual formulation is genuinely interesting. Sunday Riley's founder is a cosmetic chemist by training, and the dual retinoid approach reflects real formulation sophistication. HPR — Hydroxypinacolone Retinoate — is a retinoic acid ester that binds directly to retinoid receptors without requiring the multi-step enzymatic conversion that retinol undergoes. Retinol needs to be converted to retinaldehyde, then to retinoic acid, before it can activate receptors. HPR skips that entire chain. In vitro research has shown HPR stimulating procollagen production at levels comparable to tretinoin, with significantly less inflammatory response.

The second retinoid component — liposome-encapsulated retinol — provides a sustained-release mechanism. The phospholipid liposomes act as tiny delivery vehicles, slowly releasing retinol into the skin over hours rather than dumping it all at once. This time-release approach reduces the irritation spike that comes with free retinol while extending the duration of retinoid activity. The result is a formula designed to deliver retinoid benefits through two parallel pathways: HPR for immediate receptor activation and liposomal retinol for sustained, conversion-dependent activity.

The surrounding cast of ingredients reads like a soothing protocol designed to make this retinoid therapy tolerable for the widest possible audience. Bisabolol (chamomile-derived), prickly pear fruit extract, ginger root extract, honey extract, and CoQ10 all contribute anti-inflammatory and antioxidant support. Sodium hyaluronate crosspolymer provides longer-lasting hydration than standard hyaluronic acid. Cetylhydroxyproline palmitamide functions as a ceramide-like barrier support ingredient. Shea butter adds emollience. This is not a formula that drops retinoids onto your face and leaves you to deal with the consequences — it actively works to manage the irritation that retinoid therapy commonly produces.

The texture is a lightweight, slightly yellow-tinted lotion that absorbs quickly and feels almost weightless on the skin. One to two pumps covers the full face. It layers well under a heavier moisturizer or facial oil, which is how most retinoid serums should be used, especially during the adjustment period.

Then there's the smell. Multiple reviewers describe it as 'old rubber,' 'chemical,' or simply 'weird.' It's not a fragrance — the product is fragrance-free — but the combination of HPR, retinol, and the Dimethyl Isosorbide penetration enhancer produces an odor that some users find genuinely off-putting. It dissipates within a few minutes of application, and it's actually a positive sign that Sunday Riley chose not to mask it with added fragrance, but it's worth knowing about before your first application.

In practice, most users report visible improvements in skin smoothness and radiance within two to four weeks, with more significant changes in fine lines and dark spots emerging over eight to twelve weeks. The adjustment period — some mild flaking, possible tightness, occasional purging — is notably gentler than with many traditional retinol products, which is the whole point of the HPR-forward approach. Retinoid beginners frequently cite this as a manageable entry point into vitamin A therapy.

We should address the elephant in the room. In 2020, the FTC issued a consent agreement after finding that Sunday Riley had directed employees to post fake positive reviews on Sephora between 2015 and 2017. While the A+ launched in 2019 — after the fake review period — the scandal raises legitimate questions about the brand's relationship with honest consumer feedback. The brand has continued to operate at major retailers and the consent agreement prohibited future deceptive practices, but it's context that informed consumers should have.

The value question is straightforward: $85 for one ounce of a serum where the star active is present at 0.5% is a lot of money. The formulation is sophisticated, the delivery technology is thoughtful, and the soothing support is generous — but you are paying prestige prices, and competing HPR-based products exist at significantly lower price points. Whether Sunday Riley's specific formulation approach (dual retinoid, heavy soothing support, airless pump stability) justifies the premium over a simpler HPR serum at half the price is a personal calculus.

What the A+ does well — and it does do it well — is make retinoid therapy accessible to people who've been intimidated by it. The gentle adjustment period, the lightweight texture, the buffered delivery systems, and the army of soothing botanicals all work toward removing barriers to consistent retinoid use. For someone who's tried retinol before and given up due to irritation, or for someone who wants to start retinoids but is nervous about the adjustment, this formula is designed to meet them where they are. Whether that approachability is worth $85 depends entirely on what you value.

Formula

The Science

Hydroxypinacolone Retinoate (HPR) represents a newer approach to retinoid therapy that has generated genuine scientific interest. Unlike retinol, which must undergo a two-step enzymatic conversion (retinol → retinaldehyde → retinoic acid) before it can activate retinoid receptors, HPR is a retinoic acid ester that binds directly to retinoid acid receptors (RAR). This direct binding means it's pharmacologically active upon contact with skin cells, without depending on the conversion enzymes whose expression varies between individuals.

A 2018 study by Ruth and Mammone presented at the Journal of Investigative Dermatology found that HPR increased procollagen production in skin models at levels comparable to all-trans retinoic acid (tretinoin) — the gold standard prescription retinoid — without inducing the heightened inflammatory response that tretinoin produces. At the highest tested dose, HPR actually outperformed tretinoin in collagen staining metrics. The researchers concluded HPR is 'an effective alternative to ATRA and other less potent retinoids without the detrimental side-effects.'

A 2023 clinical trial by Wang et al. published in the Journal of Cosmetic Dermatology studied 42 women using HPR combined with retinyl propionate over 8 weeks. The combination reduced wrinkles by 8.3%, improved smoothness by 11.9%, decreased transepidermal water loss by 25.7%, and improved elasticity by 14.5-22.6% — with no adverse reactions reported. These results support the multi-retinoid approach used in the A+ formula.

More recently, a 2025 study by Shen et al. published in the Journal of Cosmetic Dermatology examined an HPR + retinol combination serum in 31 women over 8 weeks. The combination activated TGF-beta/Smad signaling pathways — key molecular cascades in collagen production — with a gene expression profile that more closely resembled HPR's activity than retinol's alone, suggesting HPR is the dominant driver of anti-aging effects even when combined with retinol.

The liposomal delivery of retinol in this formula uses phospholipid encapsulation to create a sustained-release mechanism. Research on liposomal retinol delivery has consistently shown reduced irritation potential compared to free retinol at equivalent concentrations, with comparable or improved efficacy due to the more gradual release profile.

References

1. Anti-aging effects of retinoid hydroxypinacolone retinoate on skin models — Journal of Investigative Dermatology (2018)
2. The synergistic effect of retinyl propionate and hydroxypinacolone retinoate on skin aging — Journal of Cosmetic Dermatology (2023)
3. An Innovative Serum With Retinol, Hydroxypinacolone Retinoate, Peptides, and Silybin Improves Mild Photoaged Facial Skin — Journal of Cosmetic Dermatology (2025)

Dermatologist Perspective

Board-certified dermatologists recognize HPR as a promising retinoid with a growing evidence base, though most note that the published research is still far smaller than the decades of clinical data supporting tretinoin and retinol. Dermatologists frequently recommend the A+ as a stepping stone for patients who find prescription retinoids intolerable — the gentler adjustment period can help build compliance before transitioning to stronger retinoid therapy if needed. The dual retinoid approach is viewed favorably from a formulation standpoint, and the dense soothing support addresses one of the most common reasons patients abandon retinoid therapy. However, dermatologists also note that the prestige price point puts this product in competition with prescription tretinoin, which is both more potent and often less expensive with insurance coverage.

Guidance

Launched in January 2019, the A+ was Sunday Riley's answer to the retinoid renaissance that was sweeping the prestige skincare market. The brand's founder — a cosmetic chemist by training — built the formula around Hydroxypinacolone Retinoate, a retinoic acid ester that had been generating excitement in dermatological research for its ability to deliver retinoid benefits with less irritation. The 'A+' name signals both the vitamin A ingredient class and the ambition to be a top-of-class retinoid treatment.

About Sunday Riley Established Brand (5–20 years)

Sunday Riley was founded in 2009 by cosmetic chemist Sunday Riley in Houston, Texas. The brand became one of the top independently owned prestige skincare lines in America and is sold at Sephora, Ulta, and Nordstrom. However, in 2020 the FTC issued a consent agreement after finding that the company had directed employees to post fake positive reviews on Sephora between 2015-2017, which raised questions about the authenticity of the brand's early review data.

Brand founded: 2009 · Product launched: 2019

Myth vs. Reality

Myth

The 6.5% concentration means this is a very strong retinoid product.

Reality

The 6.5% figure is a combined total of all retinoid and retinoid-like ingredients, including gentle esters and a blue-green algae extract. The most potent active — HPR — is present at 0.5%, and the retinol concentration within the liposomal blend is undisclosed. By comparison, many drugstore retinol products contain 0.3-1% pure retinol. The 6.5% figure is technically accurate but can overstate the product's potency.

Myth

HPR is as effective as prescription tretinoin.

Reality

In vitro studies show HPR stimulates procollagen production comparable to tretinoin, but head-to-head clinical trials in humans at comparable concentrations are limited. HPR is promising and significantly less irritating, but the evidence base is still smaller than the decades of research supporting tretinoin. It's a gentler alternative, not a proven equivalent.

FAQ

Common Praise

"Noticeably smoother and firmer skin within 2-4 weeks of consistent use"

"Significantly gentler than traditional retinol serums with minimal irritation"

"Lightweight lotion texture absorbs quickly without heaviness"

"Visible improvement in radiance and overall skin glow"

"Effective for retinoid beginners who can build tolerance gradually"

Common Complaints

"Unusual chemical smell frequently described as old rubber or medicinal"

"High price relative to the actual HPR concentration of only 0.5%"

"The 6.5% total retinoid claim includes weaker esters — potentially misleading"

"Some users still experience purging and flaking during adjustment"

"Price-to-potency ratio questioned versus more affordable HPR alternatives"

Notable Endorsements

Brand-cited clinical study: 90% of participants saw boosted radiance in 4 weeksFeatured in TODAY, HuffPost, and Elite Daily

Appears In

best retinoid serum best retinoid for beginners best hpr serum best anti aging serum

Related Conditions

aging hyperpigmentation dullness texture dark spots

Related Ingredients

hydroxypinacolone retinoate retinol coenzyme q10 chamomile hyaluronic acid