Sodium Hydroxide
In plain terms
Lye, a strong base used in tiny amounts to adjust the acidity of a product, where it is fully neutralized and harmless. The same chemical is what makes drain cleaners and hair relaxers caustic, so it depends entirely on concentration.
What the evidence shows
Sodium hydroxide is lye, one of the strongest common bases, and its role in a normal cosmetic could not be more different from its reputation. There it is used in trace amounts to fine-tune pH, and it reacts completely with the acids in the formula, so no free lye remains in the finished product; a moisturizer balanced to skin-friendly acidity with a touch of sodium hydroxide contains none of it as such. That is the safe, common case. The concern is concentration and pH. Concentrated sodium hydroxide is corrosive and causes chemical burns, which is why it is in drain cleaners, and some cosmetic categories deliberately use high-pH forms: chemical hair relaxers and cuticle removers work by being caustic enough to break down keratin, and relaxers in particular have been measured at skin-corrosive pH and linked to scalp damage and hair loss. The US cosmetic-ingredient panel reflects this split, clearing sodium hydroxide as safe when a formula is made to be non-irritating but capping it by use, for example under 5% in cuticle removers and to specific pH limits elsewhere. So the verdict turns on the product: a neutralized pH-adjuster is a non-issue, while a high-pH relaxer or cuticle product is where real caution belongs.
On the label
Listed as sodium hydroxide, usually last on the list when it is only a pH adjuster, where it is fully neutralized and reacts away. Where it matters is high-pH products, chemical relaxers, and cuticle removers, which are caustic by design and where careful use and protecting the skin matter.