Etidronic Acid

What it is

Etidronic acid is a chelating agent and appears in the salon wherever hydrogen peroxide is involved: in developers, colours and straightening products. It binds traces of iron and copper from tap water that would otherwise accelerate the decomposition of the peroxide. That is the same job that tetrasodium EDTA and citric acid do elsewhere.

What is remarkable is the second career of the same molecule. Under the name etidronate it was the first bisphosphonate used as a medicine against bone disease. The same substance, two fundamentally different situations: here a small amount in a product that is applied externally and rinsed off — there a measured dose that is swallowed and is meant to act inside the body.

Why

• Why the classification reads “low risk”: the molecule is strongly charged and water-soluble and penetrates the skin poorly. In its cosmetic role it is present in small amounts and is rinsed off; it is there to capture traces of metal, not to build the product.
• The comparison with the medicine belongs here, but the right way round. The effect on bone requires the substance to get into the body. Even when swallowed, bisphosphonates are absorbed only to a small extent; that is why the medicinal doses are high and tied to strict rules for taking them. A product applied externally and rinsed off is a fundamentally different situation. It shows why the question is never whether a substance can act, but in what amount it gets where.
• Short fields corrected: they called etidronic acid a “chelating active that balances pH” and credited it with reducing damage from bleaching and colouring. Three points need correcting: it is an auxiliary, not an active; a chelating agent does not set a pH value; and the statement on reduced damage is an efficacy claim that is not documented here. Function and reason were reworded in all four languages.
• The efficacy question remains interesting nonetheless. The idea behind it is plausible: free traces of metal let peroxide decompose in an uncontrolled way, and what decomposes uncontrolled attacks the fibre without lightening it. A chelating agent could reduce that share. But plausible is not documented — the statement therefore sits under the open points and not in the reasoning.
• The environmental side is different here from tetrasodium EDTA. Phosphonates such as etidronic acid are regarded in the detergent industry as its successors, but they too bind metals very firmly. Whether they actually behave more favourably in the environment has not been examined here and is not asserted second-hand.

When the assessment differs

• Occupational contact: anyone mixing and applying developer and colour daily has a considerably higher total exposure on the hands than the client has on the head.
• Irritated or broken scalp skin: with colour and straightening the scalp is under strain anyway; an additional substance carries more weight there.
• Products that stay on the skin: the assessment assumes rinse-off use.
• Ongoing treatment with bisphosphonates: the question is an obvious one, because it is the same substance. Given the properties of the molecule, no contribution is to be expected from external, rinsed-off use; that is not documented here, and for a personal judgement the treating doctor is the right address.
• Very hard water: the need for chelating agents is then higher. That explains the use but says nothing about tolerability.

Open questions

• Concentration in the specific product: not available.
• The statement that the substance can reduce damage from bleaching and colouring: not backed by data here. The mechanism is plausible, the magnitude of the effect in the finished product is open.
• Environmental behaviour of phosphonates compared with aminopolycarboxylic acids such as EDTA: not examined here.
• Uptake through the skin with external use: not backed by measurements. The judgement rests on the charge and water solubility of the molecule.
• Regulatory status: not checked; statements on use and maximum levels are not quoted second-hand.

Sources for this assessment

• Historical perspectives on the clinical development of bisphosphonates in the treatment of bone diseases, 2007. Hält fest, dass Etidronat-Dinatrium 1968 als erstes Bisphosphonat medizinisch eingesetzt wurde und dass die Stoffgruppe ursprünglich aus der Anwendung als Komplexbildner stammt. Begutachtete Fachliteratur, abgerufen am 10.09.2026.

Related substances

Applies to: Stabilisers and antioxidants

• 2,6-Dicarboxypyridine — Formulation ingredient
• Ascorbic Acid — Antioxidant
• Tetrasodium EDTA — Chelating agent

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