Salicylic acid is a scale remover. At ordinary shampoo strengths it clears scalp flakes, and with them some itch, redness, oiliness and scratching — the firmest thing known about it. The hair claim rests on an assumption, not on any study testing it alone and counting hairs. Harms are modest and mostly local. Most scalp research was company-funded. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Adherent Scalp Flaking Score | 0–1 of 4 after 4 weeks | Direct read-out of the only effect salicylic acid reliably produces |
| Hair density (trichoscopy) | No established target; change from own baseline, aiming for no decline | Distinguishes real regrowth from the impression created by cleaner hair |
| Hair shaft diameter | No established target; change from own baseline | Thinning shafts signal follicles shrinking, which scale removal does not address |
| Sebum excretion rate | No established target; a fall from own baseline | Confirms mechanical action on the follicular opening |
| Transepidermal water loss | Stable or falling relative to own baseline | Rising values flag barrier damage from over-use before visible irritation appears |
| Scalp surface pH | 4.5–5.5 | An alkaline drift accompanies barrier disruption and yeast overgrowth |
| Serum salicylate | Below 30 mg/dL; ideally undetectable | The only test that detects systemic salicylate toxicity |
| Estimated glomerular filtration rate | Above 60 mL/min/1.73 m² | Determines whether absorbed salicylate can be cleared |
Cadence: Scale and itch reassessed at 2 and 4 weeks, then every 3 months during maintenance; hair density and shaft diameter re-imaged no sooner than 6 months; serum salicylate only if symptoms of toxicity appear