From the Practice
The questions patients ask me most, organized by where they are in the regimen. Read the ones that apply to you. Skip the rest.
Most of my patients begin with a single product — typically the Vitamin Serum — and build a regimen from there over months. The questions that come up are not the same at week one as they are at month six. What feels urgent in the first two weeks (is the tingling normal? when will I see it?) gives way to different questions at month three (have results plateaued?) and then to different questions again at year one (should the regimen change?).
I wrote answers to the 20 I hear most often. They are organized by stage of the regimen. Some are grounded in the research. Some are grounded in what I see in the practice every day. All of them reflect the way I would answer in person — not the way a product page would.
If your question isn't here, write to me directly. I answer the ones that make it past the clinical Q&A.
Sections
- Getting Started (Week 1 – Month 1)
- The Science (Mechanism & Evidence)
- Application & Protocol
- The Long Arc (Month 3 – Year 2)
- Skin Changes & Concerns
- Next Steps & Regimen Evolution
Getting Started
Week 1 – Month 1
When will I see results?
The earliest visible changes — radiance, softening of surface texture — appear at 14 to 21 days of consistent daily use. Measurable improvements in fine lines and firmness require 12 weeks of continuous application. The independent clinical study documented 97% measurable improvement in fine lines at week 12, 94% in radiance and elasticity, 91% in firmness and texture. Skin does not remodel faster than collagen synthesizes. Patience is not a marketing strategy — it is a biological reality.
Is it normal to feel a brief tingle on application?
A mild, brief tingle on initial application is within normal range. It reflects acid dissociation at the skin surface and is not indicative of irritation. If the sensation persists beyond a few minutes or is accompanied by visible redness, reduce frequency to every other morning and reassess after one week. Tingling is not the same as effectiveness. Absence of tingling does not mean absence of biological activity.
Do I need to build up slowly, or can I start at full frequency?
You can start at once-daily use in the morning from day one. I do not require a ramp-up protocol for THD-Ascorbate because it is lipid-soluble and non-sensitizing at therapeutic concentration. After one week of morning tolerance, add an evening application. Some patients prefer evening application because of integration with retinol (see Application & Protocol below).
My skin looks worse in the first two weeks. Is that normal?
Transient changes in the first two weeks are not typical for THD-Ascorbate at this concentration. If you are experiencing increased dryness, redness, or breakouts, consider: are you introducing multiple new products at once, have you changed other actives (retinol, exfoliating acids, benzoyl peroxide) recently, or has your environment shifted. Isolate the Vitamin Serum by holding other variables constant for one week. If changes persist, write to me directly.
The Science
Mechanism & Evidence
Why THD-Ascorbate instead of conventional Vitamin C?
Conventional water-soluble Vitamin C sits atop the stratum corneum — the lipid-rich outer layer of skin — and does not efficiently cross it. THD-Ascorbate (Tetrahexyldecyl Ascorbate) is lipid-soluble. It crosses the stratum corneum and is activated intracellularly by esterases at the fibroblast level. The fibroblast is where collagen synthesis occurs. Without delivery to the site of action, the concentration printed on the label is biologically immaterial.
What does "therapeutic concentration" actually mean?
It means the concentration at which clinical studies document measurable tissue response — not the minimum needed to list "Vitamin C" on a product label. The Vitamin Serum is formulated with 13% THD-Ascorbate, the concentration our independent 12-week study used to document 97% improvement in fine lines. A 2% dose of any active is technically present in a formula. Whether the skin responds to it at that level is a different question.
What does THD-Ascorbate actually do at the cellular level?
Ascorbate is a required cofactor for prolyl hydroxylase — the enzyme that hydroxylates proline residues in procollagen during collagen synthesis. Without ascorbate, procollagen cannot fold into the stable triple-helix form and is degraded before becoming functional collagen. Delivering ascorbate intracellularly at the fibroblast is what enables the enzyme to do its job. This is why the mechanism matters more than the label concentration.
Is the clinical study peer-reviewed?
The study was independent and methodologically rigorous: 33 patients, ages 40 to 65, 12 weeks of consistent daily use, assessed with standardized clinical grading and instrumented photography. It has not been submitted to a peer-reviewed dermatology journal. Peer review is not a prerequisite for clinical validity — it is an additional layer of methodological vetting. The study design followed standard protocols used in industry-sponsored clinical dermatology research.
Application & Protocol
How to use, when, and in what order
Should I use the Vitamin Serum morning, evening, or both?
Morning application is the foundation. It pairs with daily sunscreen and provides the ascorbate cofactor during the window when UV and environmental oxidative stress occur. Evening application layers under other correctives — retinol, peptides — and supports overnight collagen synthesis. Once-daily morning use is sufficient to see the 12-week clinical result. Twice-daily use delivers faster cumulative improvement.
Can I layer the Vitamin Serum with retinol?
Yes. The belief that Vitamin C destabilizes retinol applies to formulations containing both actives in the same solution — not to sequential layering on skin. Apply THD-Ascorbate to clean, dry skin first. Wait 60 seconds for absorption. Apply retinol on top. The two actives do not chemically conflict at the skin surface or at the cellular level when applied separately.
How long before I layer sunscreen on top?
Sixty seconds. Sunscreen goes on top of all morning correctives. The 60-second wait is not about chemistry — it is about allowing the serum to absorb without the sunscreen mobilizing it at the surface or diluting it before it has penetrated.
Should I refrigerate the Vitamin Serum?
No. THD-Ascorbate is oxidatively stable at room temperature. Conventional water-soluble Vitamin C degrades in open air and benefits from refrigeration — that is where the refrigeration advice comes from. THD-Ascorbate does not. Store at room temperature, away from direct sunlight and prolonged heat.
The Long Arc
Month 3 – Year 2
What should I expect at month 3?
By month 3, most patients reach a plateau phase. You may stop noticing changes week-over-week because the improvement has compounded and become the new baseline. This is not the product losing effect — it is that you are no longer comparing against the starting point. The test: take a photograph now, compare it to a photograph from month 0. The difference is in the photographs, not in the mirror you look at daily.
What changes at month 6?
Collagen remodeling compounds. Skin that was previously responding to correction begins behaving differently under stress — healing faster, reacting less, recovering more predictably from environmental insult. This is where many patients shift from a correction-emphasized regimen to a maintenance-emphasized one. For some, this means adjusting the Vitamin Serum to once-daily and introducing additional correctives — pH-Optimum Retinol at night, a Replenishing Eye Serum, or a Pigment Refiner for specific concerns.
If I stop, will I lose the results?
If you discontinue THD-Ascorbate, skin returns toward its pre-intervention baseline over approximately 8 weeks as collagen turnover continues without the ongoing cofactor input. You do not lose everything — the existing collagen remains — but the regimen is no longer supporting new synthesis at the same rate. Restarting resumes the improvement curve from the new starting point. The investment is not permanently lost, but continuous daily use is biologically more efficient than discontinuous use.
Do results continue improving past year one?
The rate of visible change flattens but the underlying biology continues. Collagen synthesis is an ongoing process at the fibroblast level, and consistent daily delivery of the ascorbate cofactor continues to support it. Patients who use the regimen consistently across multiple years demonstrate measurably different skin behavior under stress — faster recovery, reduced reactivity, improved tolerance to active ingredients — than those who do not. The clinical study measured 12 weeks. The biology does not stop at 12 weeks.
Skin Changes & Concerns
Sensitivity, melasma, seasons, procedures
My skin feels more sensitive since starting. Is that normal?
For most patients, no. THD-Ascorbate at therapeutic concentration is non-sensitizing on its own. If you are experiencing new sensitivity, consider whether you are using other actives — retinol, AHA, BHA — at higher frequency than skin is tolerating, whether environmental conditions have changed (seasonal shift, travel, heat, prolonged mask wearing), and whether the sensitivity is limited to the application site or generalized. Reduce to every other morning for one week and reassess. If the sensitivity is generalized or persistent, the cause is likely not THD-Ascorbate.
I have melasma. Is this regimen appropriate for me?
Yes, with additions. The THD-Ascorbate mechanism — regulating melanocyte activity and normalizing pigment distribution — is directly supportive in melasma protocols. It is not sufficient as monotherapy. For melasma I add the Pigment Refiner (tranexamic acid and niacinamide, layered mechanism), strict daily broad-spectrum SPF 50+, and deliberate avoidance of triggering factors (heat exposure, hormonal fluctuation, certain photosensitizing medications). Melasma responds to protocol discipline, not to any single product.
Does the regimen change with seasons?
Minimally. The core regimen is seasonally stable. Winter may increase the need for additional barrier support (Essential Hydration, occlusives at night). Summer increases the importance of consistent broad-spectrum sunscreen discipline — not of the Vitamin Serum itself. THD-Ascorbate effectiveness does not fluctuate with humidity or ambient temperature.
I'm post-procedure (laser, peel, microneedling). Can I use the Vitamin Serum?
It depends on the procedure and the timeline. For surface-level treatments (microneedling, superficial chemical peels, non-ablative laser), resume Vitamin Serum 72 to 96 hours post-procedure or once epidermal integrity has returned. For surgical recovery, follow the post-operative protocol I provide specifically for your procedure — timelines vary. THD-Ascorbate supports wound healing biology through ascorbate cofactor supply. It is compatible with healing skin, not disruptive to it, when introduced at the correct timepoint.
Next Steps & Regimen Evolution
What comes after the Vitamin Serum
What's the next product in my regimen?
The answer depends on your primary concern. For fine lines, wrinkles, and firmness beyond what Vitamin Serum alone delivers: pH-Optimum Retinol in the evening, layered after THD-Ascorbate. For pigmentation and uneven tone: Pigment Refiner. For under-eye concerns — darkness, crepe texture, fine lines: Replenishing Eye Serum, twice daily, a pea-sized amount. For barrier restoration and hydration: Essential Hydration.
Should I add all of them at once?
No. The regimen is additive over months, not days. Start with the Vitamin Serum for 12 weeks. Then add one additional corrective based on your primary concern. Assess at another 12-week interval before adding the next. Skin remodeling is a stepwise process, and layering too many actives simultaneously reduces your ability to identify which product is doing what — and which adjustment, if any, is needed.
When should I adjust my regimen?
Three signals suggest an adjustment is warranted: results have plateaued for more than 8 weeks, your primary concern has changed, or your skin's behavior under stress has shifted (noticeably more or less reactive, more or less oily, a different seasonal response than prior years). These are the conversations I have with patients in the office — and the ones I can help answer by email.
Can I consult with you directly?
drkoo@drkooskincare.com is monitored personally. Send a brief description of your current regimen and your primary concern. I answer questions that make it past this clinical Q&A — the ones that need my eyes on them specifically. For a full personalized regimen, our skin consultation matches a medical-grade regimen to your skin in about two minutes.
This Q&A is updated as new questions arrive from the practice. If a question you've been carrying isn't answered here, write to me — it may be the next one added.
— Dr. Michele Koo, MD, FACS
Board-Certified Plastic Surgeon
Founder, Dr. Koo Private Practice Skincare
New to the practice?
If you haven't yet started a Dr. Koo regimen, begin with Koo Me In → — a short guide to where to start and how the products build on one another. Then return here when you're ready for the clinical detail.