A Reddit thread keeps comparing TIRTIR and SKIN1004 as if both azelaic acid serums sit at 12%, but only one of them does. Here is what each label actually says, why azelaic acid is unusually hard to formulate at high strength, and why the percentage on the front of the bottle tells you less than shoppers assume.
· 5 min read
A thread making the rounds on skincare forums asks a simple question: has anyone tried TIRTIR's azelaic acid serum against SKIN1004's version, since both keep getting mentioned in the same breath as 12% azelaic acid options. The comparison falls apart before it starts, because only one of those two products is actually labeled at that number, and the gap between what a bottle says and what it does is bigger than either brand's marketing suggests.
TIRTIR's Azelaic Acid Serum lists 12% azelaic acid on its own product page [6]. That figure is real, and it sits below the two strengths cleared for prescription use in the United States: Finacea gel at 15% [1] and Azelex cream at 20% [2]. So TIRTIR's serum is a genuinely mid-strength OTC formula, not a knockoff of a prescription product, and not an outlier among cosmetic azelaic acid launches.
SKIN1004's own listing names its product the Azelaic Acid 10 Ampoule, putting the stated concentration at 10%, not 12% [7]. The 12% figure that keeps surfacing next to SKIN1004 in forum posts and comparison roundups appears to come from shoppers pattern-matching two K-beauty azelaic acid launches together rather than reading either brand's own page. It is a two-point gap that matters if the whole reason someone is choosing between them is strength.
Finacea gel is cleared for prescription use at 15% and Azelex cream at 20%, both requiring a prescription and both regulated as drugs rather than cosmetics [1][2]. Those numbers exist because the vehicle, the exact acid form, and years of trial data back up that specific strength in that specific base. A cosmetic serum borrowing a similar-sounding percentage has not gone through that same process, which is exactly why OTC azelaic acid products cluster lower, typically in the 5% to 12% range rather than climbing toward 20%.
Part of the reason OTC percentages stay modest is chemistry, not caution. A 2025 review in the journal Pharmaceuticals notes that azelaic acid is only sparingly soluble in water, roughly 2.4 grams per liter at room temperature, and needs a fairly narrow pH window, around 3 to 5.5, along with carrier systems such as ethoxydiglycol or microemulsions to stay dissolved and actually reach skin rather than sitting on the surface [4]. Cramming a higher number onto a label does not solve that problem. A brand can print 12% or even 15% on a bottle, but if the acid is not staying in solution and moving through the formula's chosen pH and solvent system, that percentage is doing less than it appears to.
There is direct evidence that percentage and performance can diverge. A 2017 study in the Journal of Clinical and Aesthetic Dermatology compared azelaic acid formulations and found that a 15% gel penetrated skin markedly better than a 20% cream, despite the cream carrying the higher number [5]. The gel's vehicle simply moved the same ingredient through skin more effectively than the cream's did. That result is the clearest argument against shopping by percentage alone: the base matters as much as, and sometimes more than, the concentration.
Azelaic acid is a dicarboxylic acid that behaves somewhat like both an AHA and a BHA, and Cleveland Clinic describes it as especially useful for evening out the look of skin tone, since it is a larger molecule that tends to be gentler on skin that reacts easily [3]. The Pharmaceuticals review breaks its effects into four categories worth knowing before buying at any percentage: it may help calm the look of surface bacterial activity, may support a calmer appearance where skin looks reactive or congested, may help even out the look of tone by acting on pigment-producing activity, and carries antioxidant properties that can support the look of overall skin quality [4]. None of that is guaranteed by a percentage on a bottle; it depends on the acid actually being delivered into skin.
The FDA-cleared label for Finacea, which discloses trial data most cosmetic brands never have to publish, shows that in the trials supporting that label, burning, stinging, or tingling was reported at a rate of 29%, itching at a rate of 11%, dry or scaling skin at a rate of 8%, and redness or irritation at a rate of 4%, with these effects concentrated in the first several weeks and generally easing with continued, consistent use [1]. Consumer accounts of an early adjustment period with OTC azelaic acid serums echo this pattern, though it is less rigorously tracked outside of prescription trials. Anyone starting either the TIRTIR or SKIN1004 product should expect a short window where skin looks a little reactive before it settles.
The practical fix is simple: read the brand's own current product page rather than trusting a forum thread's shorthand, since labels and formulas change and secondhand comparisons calcify around old information. Beyond the number, look for how a brand describes its base, since a well-designed lower-percentage formula in the right pH range can outperform a higher-percentage one in a poorly suited vehicle. Start any azelaic acid serum a few nights a week rather than nightly, watch for the tingling window described above, and give a formula six to eight weeks before judging whether the look of tone or texture has shifted. If skin reacts sharply or the appearance changes are not what you expected, talk to your doctor before combining strong actives instead of stacking on a second exfoliating product to compensate.
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