Cysteamine HCl

What it is

Cysteamine HCl is a reducing agent and thus an active of the same kind as ammonium thioglycolate: it breaks part of the disulphide bonds in keratin so that the hair can be moved into a new shape. The subsequent fixing step closes the bonds again in their new arrangement.

The difference lies in the working range. Cysteamine works at a lower pH, so the treatment runs in a less alkaline environment. Manufacturers frequently derive from this the claim that the process is gentler. That is plausible, but it is a statement about a single parameter — the stress on the hair also depends on concentration, processing time and starting condition.

Why

• Here too: the substance works because it alters the fibre. There is no permanent reshaping without an intervention in the structure, whichever reducing agent is used.
• “Milder” is a statement about pH, not automatically about hair damage. A lower pH means less alkaline swelling — that is a real difference. Whether less damage follows in the result depends on concentration and processing time and is not backed by data here.
• Why the classification reads “increased attention”: because of the irritant potential for skin and eyes and because of the role of reducing agents in occupational skin problems in hairdressing. That concerns repeated, unprotected contact during handling — not the properly performed treatment on the client.
• Processing time is a safety parameter here as well. If it is exceeded, more bonds are broken than intended. The manufacturer's instructions on time, intermediate checks and fixing are the most important part of the application.
• Odour is not a measure of risk. Cysteamine-based systems are often advertised as lower in odour. A weaker smell says something about the volatility of the constituents, nothing about tolerability.

When the assessment differs

• Pre-damaged, previously chemically treated or bleached hair: markedly higher risk of over-processing; a strand test and the manufacturer's instructions govern.
• Broken, irritated or inflamed scalp skin: not a suitable starting point.
• Occupational skin contact without gloves: the most important differing case, as with all reducing agents combined with wet work.
• Eye contact: irritating; practically relevant during application and rinsing.
• Several chemical treatments at the same appointment: not assessed here; the manufacturer's specifications and the assessment of the hair govern.

Open questions

• Concentration and pH of the actual product: not available. Both largely determine how strongly the product acts.
• Short field “why”: it states that the substance is “milder than ammonium thioglycolate”. A direct comparison of hair damage between the two systems has not been evaluated here; what is documented is the lower working pH, not less damage. By decision of 08.09.2026 the entry remains unchanged; the point stays documented here.
• The short fields `function` and `why` describe the substance as both waving and smoothing. Both are possible in principle, but in the same sentence it is confusing. By decision of 08.09.2026 likewise unchanged; the point stays documented here.
• Regulatory status: rules on maximum levels and on professional use exist for reducing agents in reshaping products. They have not been checked here and are not quoted second-hand.
• Data on occupational skin disease specifically with cysteamine-based systems: not evaluated here. The classification follows the substance group of reducing agents.

Related substances

Applies to: Reducing agents for reshaping

• Ammonium Thioglycolate — Reducing agent for reshaping

Further substances from the same group for which a long-form assessment already exists.