The Scientist: Mort Westman, a cosmetic chemist and president of Westman Associates, an Oak Brook, Illinois, skin and hair care consultancy

The Answer: It’s definitely a bummer: You find a shampoo that totally works for your hair, and then one day, it doesn’t. Instead of bouncy, your locks are limp. Instead of sheen, you get puff. Many women experience the heartbreak of their hair getting used to a shampoo, nixing its effects. Although it’s yet to be proven in the lab, the phenomenon is probably more than a perception bias.

Before you start pouring your favorite shampoo down the drain, eliminate confounding factors. Moving to a new city or going on vacation can expose your hair to harder (or softer) water, or an uptick in damaging sun exposure, which can change the way your coif looks and feels. In the winter, all that dry air from heaters on overdrive can also fry your strands. If none of these situations applies, your problem might be buildup.

Conditioning ingredients, such as quaternium and polyquaternium, that make hair silky and manageable target areas of damaged hair. Quats, as they’re called, and similar compounds, such as silicones, found in just about every conditioning product on the market, are positively charged and are attracted to the negative charge of these damaged spots. They are “substantive,” which means that they are designed to seek out these sites and stay behind, even after rinsing, to provide lubrication and reduce static flyaway by sending a positive charge through the length of the hair shaft. The flipside of being substantive is that it can result in unwanted buildup that weighs hair down. The sudden return to lifelessness gives you the feeling that your hair is no longer responsive. The genie has left the bottle.

Fortunately, the answer is simple: Take a break. Switch shampoos for at least another week or two. You can try a clarifying shampoo or anything that is either deep-cleansing or made for oily hair. (Clarifying is basically just a fancy word for deep-cleansing anyway.) Above all, choose a product that’s made by a different company. Manufacturers tend to use the same ingredients across formulas in their various product lines, so to effectively eliminate the culprit you want to veer to a separate brand entirely. After your time apart, go back to your old love. Its effects should be as apparent as they were to begin with.

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