The extract of turmeric is obtained from the rhizome of Curcuma longa. It belongs to the family of plant-based antioxidants, within the curcuminoid group, and is one of the plants on which research has produced the most literature over the past twenty years.
The curcumin and related compounds neutralize free radicals and reduce the skin's response to irritating stimuli. In dermocosmetics, purified and decolorized forms are used, and this is not a minor detail: pure curcumin is such an intense yellow that it colors the formula and skin, and making it suitable for use in a face cream required specific refining work. This is why a turmeric cream is not yellow.
It appears in formulas for reactive skin and skin prone to redness, where it is combined with other soothing ingredients, and in those that work on evening out the complexion. It acts within the product, morning and evening, without any specific frequency of use.
A distinction is worthwhile, because turmeric is mainly discussed in another context: much of the research concernsoral intake, with dosages and bioavailability issues specific to that use. What turmeric does when applied to the skin is a separate matter, and that is what this page discusses.