Vitamin C

The two regenerate each other: vitamin C converts oxidised glutathione back to its active form, and glutathione does the same for oxidised vitamin C. The antioxidant effect holds longer than either on its own.

Vitamin C interrupts pigment as it is being formed; tranexamic acid quiets the inflammatory signal that starts the process. Useful for pigmentation that keeps returning after it fades.

Unlike retinol, bakuchiol is photostable — it can be used in the morning. Under vitamin C and sunscreen it adds antioxidant support during the hours the damage actually happens.

Both work best at different pH levels. L-ascorbic acid needs a pH below 3.5; retinol degrades faster in acidic environments. Layering them increases irritation without adding benefit. Vitamin C in the morning, retinol at night.

Two low-pH products back to back is a large irritation load for a small gain. An AHA can improve vitamin C penetration, but only skin that already tolerates both should try it. Vitamin C in the morning, the acid at night.

L-ascorbic acid and salicylic acid both need an acidic pH, and stacking them raises stinging and redness without improving either. Separate them by time of day, or choose a pH-neutral vitamin C derivative.