No, shampoo alone cannot permanently straighten hair. While some shampoos marketed as "straightening" shampoos exist, their effect is limited and temporary.
How Straightening Shampoos Work
Straightening shampoos don't chemically alter the hair's structure. Instead, they work in a few ways:
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Smoothing: They contain ingredients that coat the hair shaft, making it smoother and less prone to frizz. This can give the illusion of straighter hair, especially when combined with heat styling. As stated in multiple sources, "[They] won't fully straighten hair, but they do provide some benefit. Also, they help to prep hair, making it easier to straighten with a blow dryer or flat iron. Smoother hair straightens easier and looks shinier and less frizzy." (SELF, Reddit)
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Weighting: Some formulas contain heavier conditioning agents that weigh down the hair, making it appear straighter. This effect is temporary and washes out. A Reddit user noted that sulfates in shampoo can dry hair and make it appear straighter (Reddit).
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Preparation for Heat Styling: Straightening shampoos primarily prepare the hair for heat styling tools like flat irons or blow dryers. By smoothing the hair cuticle, they allow heat to penetrate more evenly, resulting in a straighter look with less damage. As explained on Quora, "Straightening shampoo doesn't actually make your hair straight. It just preps it to be heat styled straight, & coats it to lay more smoothly." (Quora)
It's crucial to remember that nothing can permanently change your hair's natural texture without chemical treatments like relaxers or keratin treatments. (Reddit) The effects of straightening shampoos are temporary and wash away with each shampoo.
