Actives, and what they cost you
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Actives are what customers buy and what formulators spend most of their difficulty on, because the ingredients with the strongest evidence are frequently the least stable.
What an active buys you
A reason to charge more and a reason to be chosen. It also buys you a constraint on pH, a constraint on packaging, a shorter period after opening, and a retest whenever it changes.
That trade is worth making deliberately. A product with a well-chosen active and the discipline to protect it is a strong proposition; a product with six actives at token levels is a marketing document.
Vitamin C is the worked example
L-ascorbic acid is the best-evidenced form and the most difficult. It oxidizes reversibly in aqueous solution, and the rate depends on pH and oxygen and is catalyzed by metal ions, particularly copper and iron. The degradation products, threonic and oxalic acid, do nothing for skin.
Optimum stability sits around pH 2.5 to 3.0 and it degrades above roughly pH 3.5, in oxygen, in light and in heat.
Every design decision follows from that: a chelator to tie up the metal ions, a low pH, an airless dispenser in opaque or dark glass, and a short declared life. In practice an L-ascorbic acid product lasts four to six months once opened.
Derivatives, and the honest trade
Stable derivatives such as magnesium ascorbyl phosphate, ascorbyl glucoside and tetrahexyldecyl ascorbate hold for twelve months or more, tolerate a friendlier pH, and survive ordinary packaging.
They are also less directly evidenced than L-ascorbic acid and require conversion in the skin. That is the trade, and it is a real one rather than a marketing distinction: efficacy against stability, with the customer experiencing whichever you chose.
What to promise
What you can substantiate. An active at a level with evidence behind it, protected by a package and a declared life that make it plausible at month four.
And be careful with the language. A claim that the product treats or prevents a condition moves it out of cosmetics entirely and into OTC drug regulation, which is a different world with no small-business exemption.