So what exactly is an active ingredient and how many can one safely use? Like the terms "clean" and "comedogenic,” there's no set definition for "active ingredients" in beauty. For framework, we turn to Dr. Baumann, who has written textbooks on the subject. “For me, it means these ingredients have a biologic activity or scientific method of action that has been proven,” she says. That means anything from green tea (the polyphenols of which has been shown to be rich in skin-protecting antioxidants) to retinoids (proven to inhibit collagen degradation and to promote collagen production) can be thought of as "active". As for the latter question, “there is no specific limit on the number of actives one’s skin can handle,” says Dr. Charlene DeHaven, FACEP, M.D., a pioneer in anti-aging medicine who serves as Clinical Director at iS Clinical by Innovative Skincare. “However, there is a limit on how much product activity one’s skin can handle before injury occurs and the skin objects with a reaction or complication.”
Those with more melanin-rich skin can be particularly susceptible to some complications, like hyperpigmentation. “It’s even more important in skin of color to ease your way into actives and to pair them properly, as irritation and inflammation translates into an overproduction of melanin (i.e. dark spots) in melanin-rich skin,” Dr. Obioha says. “The worst thing you can do is cause more dark spots while trying to treat dark spots which can occur as a result of too much irritation.”
Acne, dryness, flaking, congestion, and redness count as additional skin woes that can result from improperly layering ingredients, but it’s also worth noting that filler ingredients (like fragrance, dyes, and propylene glycol) can cause allergic reactions or hives in some. Same goes for some preservatives (such as thiomersal, which can be listed as merthiolate, thimerosal, vitaseptol; and to a lesser degree, parabens, according to research).