Unlike OTC retinol, which requires conversion in the skin, tretinoin is already in its active form — retinoic acid. It binds directly to nuclear receptors in skin cells, producing faster and more dramatic results than any OTC retinoid. It's FDA-approved for treating acne and photoaging, and decades of clinical research support its effectiveness for wrinkles, fine lines, hyperpigmentation, skin texture, and collagen production. If there's a single most evidence-backed topical skincare ingredient, it's tretinoin.
The cost of that potency is an adjustment period — colloquially called the 'tretinoin purge' — during which skin can become red, dry, flaky, and temporarily worse before improving. This is not a reaction; it's an acceleration of skin processes. The adjustment period typically lasts 4–12 weeks depending on concentration and skin tolerance. Most dermatologists recommend starting at the lowest available strength (0.025%) and building slowly, using the 'slow method' of buffering with moisturizer and beginning just a few nights per week.