Ingredient review
Thiodiglycolic Acid
INCI: Thiodiglycolic Acid
Thiodiglycolic acid is a strong reducing agent used in professional hair treatments to alter curl or remove hair, but it can be irritating and is not for daily skincare.
In plain English
Thiodiglycolic acid is a chemical that breaks the strong bonds inside hair, making it soft enough to reshape or remove. It's found in perms, relaxers, and hair-removal creams. Because it's strong, it can irritate skin and is usually used by professionals, not in everyday beauty products.
Review score
Safety, usefulness, and evidence
Potentially useful with some tradeoffs
The evidence base is useful, but some claims depend heavily on the formula.
Main practical flags: irritation is moderate; clogging risk is low.
- Source
- synthetic
- Evidence
- moderate
- Irritation
- moderate
- Clogging risk
- low
Quick decision guide
Useful, but context matters
Thiodiglycolic Acid can be useful, but watch for some irritation potential.
Plain-English read
Treat this as a practical screening step before you compare products that contain this ingredient.
- Step 1Start with the score, then check the irritation and clogging risk before judging Thiodiglycolic Acid.
- Step 2Use the "Best for" and "Use caution if" sections to match the ingredient to your skin, not just to a marketing claim.
- Step 3If a product stings, breaks you out, or worsens irritation, judge the finished formula and stop using it even if the ingredient scores well.
Score terms in plain English
Irritation risk
moderateCan bother some users, especially with frequent use, damaged skin, or strong companion ingredients.
Clogging risk
lowLess likely to feel heavy or contribute to clogged pores for most skin types.
Evidence level
moderateThere is useful support, but formula details and claim strength still matter.
What it is
Thiodiglycolic acid is a synthetic organic acid containing sulfur. It acts as a reducing agent, meaning it donates electrons to break disulfide bonds in hair keratin.
How it works
In hair waving or straightening, thiodiglycolic acid reduces (breaks) the disulfide bonds that give hair its shape. After the hair is physically set into a new shape, an oxidizing agent (like hydrogen peroxide) reforms the bonds to lock in the new curl or straightness. In depilatories, it weakens hair so it can be wiped away.
Pros
Effective hair restructuring
Thiodiglycolic acid reliably breaks and reforms hair bonds, giving long-lasting curls or straightness when used correctly.
Fast-acting in depilatories
In hair removal creams, it works quickly to dissolve hair at the skin surface, reducing application time.
Cons and cautions
Skin irritation risk
This ingredient can cause stinging, redness, or chemical burns if left on too long or used on sensitive skin.
Unpleasant odor
Like many sulfur compounds, thiodiglycolic acid has a strong, rotten-egg-like smell that can be off-putting.
Best for
- People seeking professional perms or relaxers under salon supervision
- Those using depilatory creams for hair removal on legs or arms
Use caution if
- Individuals with sensitive, irritated, or broken skin
- Anyone with a known allergy to sulfur-based compounds
Usage tips
Safety summary
Thiodiglycolic acid is safe for professional use when handled correctly, but it poses moderate irritation risks. It should not be used on damaged skin or near eyes. Always follow safety guidelines and rinse thoroughly.
Research notes
Research confirms its efficacy as a reducing agent for hair restructuring. Studies note potential for skin irritation at high concentrations, but no serious long-term health risks have been identified in cosmetic use.
Common label clues
- Typical concentration
- 5–10% in professional-use products; lower in consumer formulas
- Regulatory status
- Approved for use in cosmetics in the EU and US at concentrations up to 10% in professional products; consumer products typically contain lower levels.
- Common uses
- Permanent hair waving solutions, Hair straightening treatments, Depilatory creams
- Environmental note
- Synthetic production; may contribute to wastewater sulfur load if not properly treated.
Good to know
- Thiodiglycolic acid is often used in professional salon products rather than at-home kits due to its potency.
- It is different from glycolic acid (an AHA used in skincare exfoliation) and should not be confused with it.
Common questions
What is Thiodiglycolic Acid in beauty products?
Thiodiglycolic acid is a chemical that breaks the strong bonds inside hair, making it soft enough to reshape or remove. It's found in perms, relaxers, and hair-removal creams. Because it's strong, it can irritate skin and is usually used by professionals, not in everyday beauty products.
What does Thiodiglycolic Acid do in a beauty product?
In hair waving or straightening, thiodiglycolic acid reduces (breaks) the disulfide bonds that give hair its shape. After the hair is physically set into a new shape, an oxidizing agent (like hydrogen peroxide) reforms the bonds to lock in the new curl or straightness. In depilatories, it weakens hair so it can be wiped away.
Is Thiodiglycolic Acid safe for most people?
Thiodiglycolic acid is safe for professional use when handled correctly, but it poses moderate irritation risks. It should not be used on damaged skin or near eyes. Always follow safety guidelines and rinse thoroughly.
Who should be careful with Thiodiglycolic Acid?
Individuals with sensitive, irritated, or broken skin Anyone with a known allergy to sulfur-based compounds
Research sources
Ingredient reviews are educational and are not medical advice. Patch test new products and ask a licensed clinician about persistent irritation, allergies, pregnancy-specific questions, or diagnosed skin conditions.