Zinc Pyrithione: Banned Despite Scientists Saying It's Safe

The Numbers That Should Have Saved It

In March 2020, the EU's Scientific Committee on Consumer Safety (SCCS) published their final opinion on Zinc Pyrithione. Their conclusion? Safe for use as an anti-dandruff agent in rinse-off hair products at up to 1%.

The safety margins weren't just adequate - they were exceptional:

For comparison, many approved cosmetic ingredients have margins of safety between 100-500. A MoS of 2000+ is essentially a dream scenario for a toxicologist.

So Why the Ban?

The answer lies in a classification that happened in September 2018, when the ECHA Risk Assessment Committee classified Zinc Pyrithione as:

• Repr. 1B (H360D) - "May damage the unborn child"
• STOT RE 1 (H372) - Causes damage to organs through prolonged exposure
• Acute Tox. 3 (H301) - Toxic if swallowed
• Acute Tox. 2 (H330) - Fatal if inhaled

Under the EU Cosmetics Regulation (EC No 1223/2009), substances classified as CMR (Carcinogenic, Mutagenic, or toxic to Reproduction) Category 1A or 1B are automatically prohibited in cosmetics - unless they meet all the criteria in Article 15(2):

1. Food safety compliance (for oral exposure)
2. No suitable alternatives available
3. Used for a particular purpose with known exposure
4. Evaluated as safe by SCCS

ZPT passed criterion 4 with flying colors. But criterion 2 was fatal: suitable alternatives exist.

Hazard vs. Risk: The Philosophical Divide

This case perfectly illustrates the difference between hazard-based and risk-based regulation:

The EU's approach is sometimes called the "Precautionary Principle" - when there's scientific uncertainty about potential harm, err on the side of caution. Critics call it "hazard-based overreach."

The Reproductive Toxicity Data

The CMR 1B classification wasn't arbitrary. Animal studies did show developmental effects:

The critical nuance: these effects occurred at oral doses. In topical studies - the actual route of cosmetic exposure - no reproductive effects were observed even at doses far exceeding cosmetic use.

This is where dermal absorption becomes crucial. The SCCS used a conservative 1% dermal absorption value. Combined with the short contact time and rinse-off nature of shampoos, systemic exposure is minimal.

Why It's Really About the Pyrithione

The neurotoxicity that drives much of ZPT's concern isn't from the zinc - it's from the pyrithione moiety. After absorption, zinc is cleaved off, and the pyrithione compound causes the toxic effects.

Studies with sodium pyrithione (no zinc) show the same neurotoxicity profile:

• Hindlimb weakness and paralysis
• Sciatic nerve fiber degeneration
• Spinal cord degeneration
• Peripheral retinal atrophy

The NOAEL for these neurotoxic effects (0.5 mg/kg bw/day) became the critical value for the safety assessment.

The US Continues Business as Usual

While the EU enacted its ban, the FDA's position hasn't changed:

The Cosmetic Ingredient Review (CIR) deferred their evaluation specifically because FDA had already assessed it for OTC drug use. Their reasoning: if FDA approved it for drug use, cosmetic use is implicitly acceptable.

What About Allergies?

ZPT is actually remarkably well-tolerated. Positive patch test rates range from 0.2% to 1.2% - quite low for a preservative/active that's been used for decades.

There are case reports of allergic contact dermatitis, including one notable case where it triggered pustular psoriasis via the Köbner phenomenon (Jo et al., 2005). But considering the billions of applications globally, these remain rare.

The Environmental Angle

One often overlooked aspect: ZPT is very toxic to aquatic life (Aquatic Chronic 1, H410). In marine environments, it can transchelate to copper pyrithione, which may persist longer than the zinc form.

EC50 values for pelagic organisms range from 1.6-60 nM - extremely potent. While ZPT photodegrades rapidly in sunlight (half-life 7-45 minutes), it remains stable in dark or low-light conditions.

This environmental concern wasn't the stated basis for the ban, but it's worth noting for the complete picture.

The Industry Response

The March 2022 deadline forced rapid reformulation across the EU market. The main replacements:

• Piroctone Olamine became the dominant choice - similar efficacy profile, no CMR classification
• Climbazole for some premium formulations
• Combination systems using multiple lower-efficacy actives

Industry estimates suggested €50-100 million in reformulation costs across the sector, though this figure is unverified.

The Bottom Line

Zinc Pyrithione's EU ban represents the most striking example of hazard classification trumping risk assessment in cosmetics regulation. Key takeaways:

• The science said it was safe - MoS exceeded 2000 for typical use
• The classification triggered automatic prohibition - CMR 1B means banned unless exempted
• Alternatives existed - this prevented the exemption pathway
• 50 years of safe use didn't matter - historical safety data doesn't override CMR classification
• The US disagrees - still available at 2% under OTC monograph

Whether you view this as responsible precaution or regulatory overreach depends on your philosophical stance. The EU prioritizes eliminating potential hazards; the US prioritizes real-world risk assessment.

For consumers in the EU, the practical impact is minimal - alternatives work well for dandruff control. For the cosmetics industry, it's a reminder that CMR classification is effectively a death sentence for any ingredient that has alternatives, regardless of actual safety margins.

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References

1. SCCS (2020). Opinion on Zinc Pyrithione (ZPT) - Submission III. SCCS/1614/19. European Commission. ec.europa.eu
2. European Commission (2021). Commission Regulation (EU) 2021/1902 amending Annexes II, III and V to Regulation (EC) No 1223/2009. Official Journal of the European Union.
3. ECHA (2018). RAC Opinion on harmonised classification and labelling of zinc pyrithione. Adopted September 14, 2018.
4. Holmes AM, Kempson IM, Turnbull TL, Payne KR, Roberts MS (2018). Imaging the penetration and distribution of zinc and zinc species after topical application of zinc pyrithione to human skin. Toxicology and Applied Pharmacology, 343:40-47. DOI: 10.1016/j.taap.2018.02.012
5. Rush AK, Nash JF, Smith ED, Kasting GB, Winkle JL (2019). Formulation and Artificial Sebum Effects on the Percutaneous Absorption of Zinc Pyrithione. Skin Pharmacology and Physiology, 32(4):224-234. DOI: 10.1159/000499477
6. Reeder NL, et al. (2011). The antifungal mechanism of action of zinc pyrithione. British Journal of Dermatology, 165(Suppl 2):9-12. DOI: 10.1111/j.1365-2133.2011.10571.x
7. Jo JH, Park JY, Kim KH, Kim MG, Koh KH (2005). Pustular psoriasis and the Köbner phenomenon caused by allergic contact dermatitis from zinc pyrithione-containing shampoo. Contact Dermatitis, 52(3):142-143. DOI: 10.1111/j.0105-1873.2005.00528.x
8. FDA (2021). OTC Monograph M032 - Drug Products for the Control of Dandruff, Seborrheic Dermatitis, and Psoriasis. 21 CFR Part 358.