Pyrithione Zinc
Dandruff is caused by a yeast infection. Yeast eat your scalp secretions and metabolize them into oleic acid. Oleic acid in turn irritates your skin, which sparks an increase in cell production, and thus an increase in dead skin cells (though only if you are predisposed to dandruff — some people don't react to oleic acid). Pyrithione zinc depolarizes yeast cell membranes, which kills them.
Polyquaternium-10
For something with a name like a sci-fi planet, this ingredient is pretty humdrum. It's a moisturizer (it helps provide the "dry scalp care") and an antistatic agent (now that your pate is free of dead skin, you don't need to look like a live wire).
Zinc Carbonate
A mineral formerly called calamine and now called smithsonite (after the man whose money was used to found the Smithsonian), this astringent was used in the old days to dry up skin lesions. Here, it ensures that the zinc and pyrithione stay mixed together and effective.
Sodium Laureth Sulfate
A detergent and foaming agent common in shampoos. The detergent lifts grease and dirt off your scalp and hair, but the foam is cosmetic — people just like foamy cleansers.
Dimethicone
There is natural oil on your hair, and when you wash it all away, your follicles are clean, dry ... and unprotected. Dimethicone is a silicone-based oil — you might know it as the squishy stuff inside breast implants — that conditions and protects the hair until its natural oils build up again.
Glycol Distearate
This waxy substance adds a milky pearlescent quality to the shampoo. It does nothing to clean hair, but it is psychologically vital: According to Head & Shoulders maker Procter & Gamble, opaque shampoos are perceived by the public as gentle.
__Magnesium Sulfate and
Magnesium Carbonate Hydroxide__
P&G tells us that these are excipients, substances that help the active ingredient, pyrithione zinc, work more effectively. In this instance, the excipients probably keep the pyrithione zinc well suspended in the solution and encourage it to home in on your scalp.
__Methylchloroisothiazolinone
and Methylisothiazolinone__
These two preservatives kill any bacteria that form in the bottle. In a test tube, methylisothiazolinone (MIT) has been proven to kill neurons, but don't worry: Even the European Union, which is generally much stricter than the FDA, agrees that MIT is safe in shampoo. So that should be a weight off your shoulders.
*Head & Shoulders Dry Scalp Care Pyrithione Zinc Dandruff Shampoo
