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The Apothecary Os

Routine science

Active Stacking & Compatibility

Build an AM/PM stack tuned for melanin-rich skin. Catches incompatibilities, pH conflicts, photosensitivity, and post-inflammatory hyperpigmentation (PIH) triggers before you formulate.

Your stack

pH 5–7 • 2–10% • PIH 0/3
pH 4.5–6.5 • 2–5% • PIH 0/3
pH 5–6.5 • 0.1–1% • PIH 2/3
pH 4–5 • 5–20% • PIH 0/3

AM application order

  1. 1.Niacinamide5% • layer 3
  2. 2.Tranexamic Acid3% • layer 3

PM application order

  1. 1.Retinol0.3% • layer 4
  2. 2.Azelaic Acid10% • layer 4

Findings

  • ⚠ PM: Retinol has an elevated risk of dark marks (PIH) on deeper skin tones (Fitzpatrick IV–VI). Patch test; buffer with niacinamide/ceramides.
  • ✗ Using exfoliants/retinoids without sunscreen — high risk of dark marks on deeper skin tones. Pair with a tested, labeled broad-spectrum sunscreen, SPF 30 or higher.

PIH (post-inflammatory hyperpigmentation) is the #1 reason actives fail on Fitz IV–VI skin. When in doubt: lower strength, alternate nights, buffer with niacinamide + ceramides, and always finish AM with a tested broad-spectrum sunscreen, SPF 30 or higher.