Treatment-heavy means buffered: don't apply vitamin C and retinol on the same day, don't stack BHA with retinol. Mixing creates irritation that erases gains. The routine assumes a baseline of healthy barrier function — if you're flaring or sensitized, run the Sensitive Reset for 6 weeks before introducing actives. Timeless Vitamin C + E + Ferulic mirrors the Skinceuticals C E Ferulic formula at a tenth of the price (Skinceuticals isn't sold on Amazon). The COSRX RX retinol cream is the cheaper retinol alternative.