If you stockpile blotting papers and face powder, then you might also need to shampoo every day. You should also avoid a low-poo regimen. Women with normal or dry skin, however, can skip washing for a day or two.

Fusco says the biggest problem with oily hair is the most practical one: It smells. In addition to causing odors, an oily scalp undermines hair's health and strength.

"Oily hair is not ideal," Kingsley says. "If left to set on the scalp, oils can start to smell and can irritate the skin... And a healthy, clean scalp is vital to healthy hair growth."

Some dry shampoos, like Living Proof Perfect Hair Day Dry Shampoo, are specifically formulated to bind to oil, sweat and odors so they can be brushed out of hair. Although they can't rid your hair of dirt or styling products, these dry shampoos are OK to uses a couple times a week on days you don't lather up.

(Got a dry shampoo habit you can't quit? Read on to find out why you should exfoliate your scalp.)

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If your scalp is flaky, you might blame it on dryness and assume it's better to skip shampoo for a few days. And you'd be so wrong.

"Individuals with oily hair and dandruff benefit from more frequent shampooing, which removes oil [and] dead skin flakes," says Fusco. She recommends looking for shampoos with zinc pyrithione, one of the strongest dandruff-fighting ingredients on the market. Try: Neutrogena T/Gel Daily Control 2-in-1 Dandruff Shampoo Plus Conditioner, $6.99.

"For those who already have scalp issues, such as dandruff, daily shampooing is essential," agrees Kingsley. "These dead skin cells need to be removed daily." Hair loss is also a risk of untreated dandruff, she says. "A flaky scalp is proven to cause and/or worsen hair shedding."

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One of the most important questions to ask yourself when considering how often you should wash your hair: How often do you want to wash your hair?

If you can't stand to go more than a day or two without reaching for the shampoo bottle, it's cool. "With the vast array of shampoos and conditioners available for every hair type, it is perfectly fine to shampoo daily," says Fusco. Tanzi is a fan of every-other-day shampooing, suggesting "three or four times a week" is acceptable for most hair types.

If you swear your hair is better behaved with infrequent shampooing, that's also fine. Just make sure to wash twice a week. "I don't recommend going any longer than three or four days," says Blaisure. "This is likely to cause scalp problems."

And definitely don't expect a trichologist to sign off on your no-poo routine. "After all," says Kingsley, "You take your hair and scalp to the same places you take your face, and they get just as dirty by the end of the day."

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