Mercaptoacetic acid, used in hair waving and depilatory products and regulated in Annex III.
What it is
Thioglycolic acid is mercaptoacetic acid, a small organic acid with a thiol group that can break disulfide bonds in keratin. CosIng lists it as an antioxidant, depilatory, hair waving or straightening and reducing ingredient. In the EU it is covered by an entry in Annex III (substances subject to restrictions).
Where it is used
Appears on the ingredient lists of a small number of products.
Limits of interpretation
The INCI list shows which ingredients are present, not how much of each. Above 1%, ingredients are listed in descending order of weight; below 1% the order is free.
See the EU regulation section for the applicable limits, conditions and label warnings, which differ between hair products, depilatories, hair rinse-off products and eyelash waving products.
Functions
- Antioxidant
- Inhibiting reactions promoted by oxygen, thus avoiding oxidation and rancidity.
- Depilatory
- Breaking down the the mechanical strength of hair fibers so that they can be removed by mild scraping/rubbing.
- Hair waving or straightening
- Modifying the chemical structure of the hair, allowing it to be set in the style required (permanent waves or hair straightening).
- Reducing
- Changing the chemical nature of another ingredient by adding hydrogen (or removing oxygen).
EU regulation
- Product type
- (a) Hair products (b) Depilatories (c) Hair rinse-off products (d) Products intended for eyelash waving
- Maximum concentration
- (a) (i) 8% (ii) 11% (b) 5% (c) 2% (d) 11% The abovementioned percentages are calculated as thioglycollic acid
- Conditions and warnings
- Conditions of use: (a) (b) (c) (d) Avoid contact with eyes Rinse eyes immediately if product comes into contact with them. (a) (c) (d) Wear suitable gloves Warnings to be printed on the label: (a)(i) (b) (c) Contains thioglycolate Follow the instructions Keep out of reach of children (a)(ii) (d) For professional use only Contains thioglycolate Follow the instructions’
- As of
- 3 November 2026 · (EU) 2015/1190
Safety Gate alerts (EU)
2 alerts about dangerous cosmetics in the EU mention this substance. An alert concerns a specific product, usually one that breaks EU rules.
- Ardell Professional – Brow Lamination (2025-05-02)
- MCCM – T50T35 Professional Use Peel (2022-02-01)