Scenario You get a brand new serum and fall in love at first sight. The serum does wonders for your fragile skin, giving it a radiance that transforms your complexion. Enthralled, you diligently use the bottle for several months. But with each turn of the calendar page, you notice that your skin is less radiant and less special. Is it your imagination, or have you become immune to skin care?

According to dermatologist Marnie Nussbaum, M.D., some ingredients in skin care products may weaken over time through tachyphylaxis, a term that refers to eventual tolerance to a drug. When that happens, "the body alters signaling pathways at the molecular level so that the product no longer works."

And, "the body can't do anything about it.

But wait. Don't throw out all your beauty products just yet: "Tachyphylaxis is very uncommon in general skin care and usually only occurs with topical steroids in conditions like psoriasis and eczema," says Dr. Nussbaum. There are many factors that contribute to why a product might feel less effective."

According to Dr. Nussbaum, the strongest response is always immediately after the first use of a product. This means that over time, even though the product is still working, the visible effects may be weaker and less noticeable.

Furthermore, people who repeatedly switch skin care products may be using a mixture of incompatible ingredients. For example, "Benzoyl peroxide should not be used with vitamin C because it oxidizes it. Vitamin C and retinoids are very irritating when used together and should be used separately (morning and evening)," Dr. Nussbaum advises.

Environmental factors may also be to blame. As the seasons change and skin dries out, it becomes more difficult for serums and lotions to get through dead skin cells. Changes in the external environment, such as cigarette smoke, household chemicals, or increased exposure to sunlight, can increase free radical damage to the skin. Cosmetics therefore have to do double duty and appear less responsive"

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There are also special ingredients, like retinol. Dr. Nussbaum says: "Most people think retinoids stop working over time, but your skin has just developed a tolerance to that ingredient. You should expect no redness or peeling."

Listen to your complexion. According to celebrity facialist Joanna Vargas, founder of Joanna Vargas Salon and Skincare in New York and L.A., "Products don't change, but skin does. Therefore, the products you should use need to change over time."

She adds, "The products you use need to change over time.

With the change of seasons, you may want to switch up your skincare routine, Vargas says. This includes switching from serums to lotions or from lotions to creams during the dry winter months.

Nevertheless, Dr. Nussbaum reiterates that when adding new ingredients, one must be careful of bad product combinations. He says, "It is wise to separate powerful ingredients, one in the morning and the other in the afternoon, so that they are not combined with each other. That way, you get the benefit of all the active ingredients without worrying that the powerful ingredients will cancel each other out or that you will put on too much at once and get a hairball"

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According to Dr. Nussbaum, exfoliation should be performed once or twice a week to remove dead skin cells and allow the active ingredients to penetrate deeper. For best results, always remove makeup and thoroughly cleanse before applying anything.

"It is also important to combat free radical damage. Free radicals slow skin cell turnover, resulting in dry, dull skin and an increase in fine lines and wrinkles," says Dr. Nussbaum. As always, apply a broad-spectrum sunscreen with SPF 30 or higher year-round.

Avoid leaving bottles in a humid bathroom, as some ingredients may become unstable over time (e.g., opening and closing the lid daily), weakening the effectiveness of the product. The active ingredients may separate or decompose over time and depending on how the product is stored. Always store product in a cool, dry place and close the lid immediately after use.

Most importantly, do not play skin care bartender. A physician can streamline your skin care routine with an individualized approach that best suits whatever your needs are.

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