Read This Before You Buy A Cheap Retinoid

Photographed by Megan Madden.
It's unheard of for a single skin-care product to fly off shelves at the same rate as, say, a KKW Beauty contour kit. But that's exactly what happened last week, when Kim Kardashian West, she of the dewy, poreless skin and poor judgment calls, shared with subscribers of her paywalled website that she relies on a £8 serum to slow down the signs of ageing.
The Ordinary's Granactive Retinoid* 2% Emulsion sold out at Sephora shortly thereafter, another nail in the coffin for retinoids' former reputation for being stodgy anti-ageing ingredients that are either prohibitively expensive or only available by prescription. Times have changed; celebrities now charge you money just to look at their websites. But some things remain the same — like what, exactly, retinoids are in the first place.
Retinoids are a class of vitamins, all derived from vitamin A, under which falls, in order of potency: retinoic acid, retinaldehyde, retinol, and retinyl palmitate. Retinoic acid is the active form of the chemical, the one that can immediately be used by the skin; it almost always requires a prescription, like in Retin-A (tretinoin), cosmetic chemist Randy Schueller of The Beauty Brains tells us. The others are slightly less effective than the real thing because the skin must convert them before they can be utilised, and you lose some of the potency in the process.
Retinyl palmitate, sometimes called vitamin A palmate, is the least powerful retinoid. That's why it's also the cheapest... and, on the plus side, the least drying. "The most 'effective' forms tend to be the most irritating," says Schueller, so a weaker form doesn't necessarily mean a bad thing. "Some people get better results using a less effective form because it doesn't irritate their skin as much, so they're more likely to use it more often." But even if retinoids are the sweet, sweet anti-ageing, anti-acne ambrosia the world has been waiting for but doesn't have totally figured out just yet, one cannot simply walk into a store and grab the nearest thing that says "retinol" or "vitamin A" on it.
Or, well, you can, but it's probably not the best idea. You'll want to take into account your skin type — many sensitive types are too delicate for the full-strength stuff and can do more harm than good with dryness and irritation if they dive into it too fast — and you'll also want to be extra cautious about only purchasing from brands you trust, not just the cheapest formula or the one that promises to be the most potent. (You'll also want to not be pregnant. There is no reported evidence of topical retinoids causing any harm to an unborn child, but some studies have shown that high doses of vitamin A can be harmful, and oral retinoids like Accutane are known to cause birth defects.)
As celebrity aesthetician Renée Rouleau explains, retinol is known to be highly unstable — not only extremely fragile to oxygen and light exposure (which is why you should look for ones that come in opaque, airless packaging), but also when combined with other ingredients together within a product. "Simply put, retinol doesn't play well with others, and only a very experienced formulator will know the best way to make it effective," she says. "Anyone can add the ingredient retinol to a product and market it for anti-ageing, but it may not be doing much of anything."
And, yes, a well-formulated, high-quality retinoid can err on the expensive side to formulate, which is how they got that reputation in the first place. But you don't need to spend tons of money to get a good, effective retinoid — you just need to know what to look for, and where. How's that for making good judgment calls?