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A dermatologist reveals how to keep your hair healthy and strong.

Some weeks, we confess we've washed our hair twice in one day. Other times, we've gone (gasp) four or five days without washing our hair at all. What's the magic frequency?

There isn't. How often you wash your hair actually depends on a variety of conditions, as Dr. Anthony Rossi, MD, of New York City's Memorial Sloan Kettering Cancer Center explains. Read on for his top tips for washing your hair and overall hair health.

1. Focus on scalp health.

“Maintaining a nourished, healthy scalp all depends on the type of scalp you have. If you have a dry, flaky scalp than you may not want to wash it too frequently because regulars soaps and shampoos will dry the scalp skin and hair," says Dr. Rossi. "If you exercise frequently but still have a dry scalp you can rinse the hair with tepid water to get rid of the sweat and sebum [excess oil] but you don't necessarily need shampoo every day."

WATCH: Are You Washing Your Hair The Right Way?

2. Try this rule of thumb.

"On average I tell patients to wet their scalp and hair daily, but only wash it two to three times per week if needed," says Dr. Rossi. "Again, depending if you have a normal amount of scalp oil. If excessively oily you may need to wash more."

3. Treat yourself to a cleansing conditioner.

"As a regular cleanser, using a cleansing conditioner that is sulfate-free [is] helpful. It will not dry out the hair like regular shampoos that contain harsh sulfates. It won’t over-dry the hair, so if you do like washing your hair more often you can use it more," says Dr.Rossi, who likes WEN cleansing conditioners.

4. Eat well.

"Hair health is important and many patients ask if there is a diet that can help. Maintaining a healthy balanced diet does affect your hair as undernourished people will have dry brittle hair that tends not to grow." Talk to your dermatologist or doctor to see if a supplement may be right for you.