Which ingredient is most commonly found in products that completely straighten hair?

Sodium hydroxide is the ingredient most commonly found in products that completely straighten hair. This chemical is a strong alkaline agent that alters the hair’s structure by breaking down the disulfide bonds in the hair shaft. When sodium hydroxide is applied to the hair, it raises the cuticle, allowing for the chemical to penetrate and effectively relax or straighten the hair.

This process is often referred to as a "lye relaxer" treatment. The hair becomes smoother and can be styled more easily, as the natural curl or wave pattern is significantly reduced.

Other ingredients listed, such as ammonium thioglycolate, are generally used for perming or creating curls rather than straightening. Cysteamine is a less common alternative that is sometimes found in milder relaxer formulations but is not the primary ingredient for complete straightening. Methacrylic acid is primarily used in the creation of acrylic nails and is not relevant to hair relaxing treatments. Thus, sodium hydroxide stands out as the most effective and commonly used ingredient in hair straightening products.