Mercaptopropionic Acid: What Is It, Cosmetic Uses, Benefits & Side Effects

Curious about this ingredient? In this article we're explaining exactly what it is and why it's used within cosmetic formulations.
Updated on: July 1, 2025
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We verify all information on this page using publicly available nomenclature standards from The Personal Care Products Council (PCPC), the European Commission's CosIng database and documentation provided directly by ingredient manufacturers. Our analysis is based on technical data from these sources to ensure accuracy and reliability.

What Is Mercaptopropionic Acid?

Mercaptopropionic acid is a small sulfur-containing molecule also known as 3-mercaptopropionic acid. It belongs to a family of compounds called thiols, which means it carries a sulfur atom that can break certain chemical bonds in hair. The ingredient is usually made in a lab through a reaction that brings together acrylic acid and hydrogen sulfide, followed by careful purification to remove unwanted by-products.

Chemists first took interest in mercaptopropionic acid in the mid-20th century while searching for gentler ways to reshape or remove hair. Its ability to reduce disulfide bonds—the strong links that give hair its structure—made it valuable in depilatory creams and home perm solutions. Over time, the industry refined its concentration and paired it with soothing agents so it could be used safely on skin and hair.

Today you are most likely to find mercaptopropionic acid in depilatory lotions, creams and gels that dissolve unwanted body hair. It also appears in some at-home or salon hair-straightening and waving products where it helps alter the natural curl pattern. A few specialty exfoliating masks use very low doses to help loosen dead surface cells, but its main roles remain hair removal and reshaping.

Mercaptopropionic Acid’s Cosmetic Benefits/Uses

Below are the key ways the ingredient improves beauty formulas

  • Depilatory: It weakens and breaks the cystine links in keratin, allowing body hair to wipe or rinse away without shaving. This leaves skin feeling smoother for longer than simple razors
  • Hair waving or straightening: By reducing disulfide bonds inside the hair shaft, it lets stylists reshape strands into curls or smooth them out. Once the desired form is set, a neutralizer reforms the bonds to lock in the new look
  • Reducing agent: In broader formulation work it serves as a mild reducer that can adjust pH or assist other active ingredients that need a controlled reduction step, improving overall product performance

Who Can Use Mercaptopropionic Acid

Crems and gels that rely on mercaptopropionic acid are generally fine for normal, oily or dry skin that is intact and free from cuts. Sensitive skin, eczema or psoriasis can react more easily because the ingredient is strong by design and works by breaking protein bonds. If your skin barrier is already compromised it may sting or redden more than usual so caution is advised.

The molecule is made in a lab from simple feedstocks and no animal material is needed so the raw ingredient itself is suitable for vegans and vegetarians. Always check the full product label though as added moisturizers or fragrances could come from animal sources.

Data on use during pregnancy or while breastfeeding is limited. The ingredient works on the outer layer of skin and hair with very little chance of entering the bloodstream yet it is still best to get a doctor’s approval before adding any depilatory or hair-shaping product to your routine when pregnant or nursing. This is not medical advice and individual guidance is essential.

Mercaptopropionic acid is not known to make skin more sensitive to sunlight. Still, freshly hair-removed skin may feel tender so daily sunscreen is smart. No major interactions with other skincare ingredients are reported although applying strong acids, retinoids or exfoliants on the same area right before or after could increase irritation.

Potential Side Effects/Adverse Reactions

Responses to topical mercaptopropionic acid vary from person to person. The points below list potential side effects that could occur yet most users will not experience them when the product is formulated and used as directed.

  • Temporary stinging or burning while the product is on the skin
  • Redness or irritation lasting a few hours after rinsing
  • Contact dermatitis including itching or small bumps in sensitive individuals
  • Chemical burn if the cream is left on far longer than the instructions state
  • Sulfur odor that can irritate eyes or nasal passages in poorly ventilated areas
  • Allergic reaction such as swelling or hives, although uncommon
  • Over-processed hair becoming dry or brittle when used in waving or straightening kits

If any troubling reaction occurs stop using the product immediately rinse the area with cool water and seek medical advice

Comedogenic Rating

Rating: 0/5 (non-comedogenic)
Mercaptopropionic acid is a small water-soluble molecule that rinses off easily without leaving an oily or waxy residue that could clog pores. It also has no known affinity for the skin’s sebum so it does not sit inside follicles or trap dead cells. Because of these traits formulators classify it as non-comedogenic, meaning it is very unlikely to trigger blackheads or whiteheads.

Bottom line: suitable for acne-prone or breakout-prone skin.

One extra note: most depilatory products using this acid contain softening agents and fragrances that may have their own pore-clogging potential, so the overall formula—not the acid itself—determines whether a product is truly non-comedogenic.

Summary

Mercaptopropionic acid acts primarily as a depilatory agent, a hair waving or straightening aid and a general reducing agent. It breaks the disulfide bonds in keratin, which lets body hair dissolve for easy removal and allows stylists to reshape hair before rebonding it into a new pattern. Its reducing power can also fine-tune pH or activate other ingredients in a formula.

In the grand scheme of cosmetic ingredients it is fairly niche, showing up mainly in depilatory creams and some at-home hair texture kits rather than everyday skin care. You will not find it in most moisturizers or serums but it remains a go-to choice when fast, inexpensive chemical hair removal is needed.

When used as directed it has a solid safety record with only minor irritation risks for sensitive skin, yet it is still a strong active. As with any potent ingredient it is wise to do a small patch test the first time you try a new product containing mercaptopropionic acid just to make sure your skin handles it well.

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