Love hair products? Then you also better love shampooing. "Individuals who use a lot of product -- hair spray, gels, et cetera -- may require more frequent shampooing," Fusco confirms. Product build-up can make hair look dull and can irritate the scalp with prolonged exposure, just like oil does.

And if you haven't given much thought to scalp condition, you should, according to Blaisure. "Researchers and scientists who study hair and scalp are realizing the importance of a balanced scalp and how it relates to hair and scalp health," says Blaisure. A healthy scalp produces soft, manageable hair that grows quickly; an unhealthy scalp can lead to unruly hair texture, sluggish hair growth and the aforementioned hair loss.

(For more proof that an out-of-whack scalp could be causing lackluster locks, read The Real Reason You're Having a Bad Hair Day.)

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When deciding how often to shampoo, "it is important to consider how damaged your hair is," says Cristina B. Although hair naturally contains a small amount of water, completely saturating your hair in water weighs it down and stretches the fibers. Healthy hair is capable of handling this stress without breaking, but hair that's been over-processed (from bleaching or coloring) is more likely to snap in these conditions, particularly when handled or brushed. So, minimizing the time treated hair is wet -- like when you're shampooing -- can prevent breakage.

Not to mention shampooing can dull the color you've worked so hard to achieve, Blaisure says. "Water and shampoo can fade your hair color the more often you shampoo, especially the reds and delicate shades of blonde that are currently so popular."

Consider the damage caused by your styling routine, too; if you heat-treat or perm your hair, you should wash a few times a week with a product made for your hair type, like Bumble and Bumble Mending Shampoo, $31.

Dry shampoo can help absorb oil on your off-days, but don't treat it as a stand-in for shampoo and water. Build-up from dry shampoo can inflame the scalp and possibly lead to hair loss.

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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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