What's the Difference Between Pyrithione Zinc and Salicylic Acid When Treating Scalp Buildup?

Pyrithione zinc and salicylic acid are both found in over-the-counter dandruff shampoos, but they work in different ways.

Pyrithione zinc: an antifungal

Pyrithione zinc (zinc pyrithione, ZPT) slows the growth of Malassezia yeasts, which are associated with dandruff. Laboratory studies by Procter & Gamble scientists found that it raises copper levels inside yeast cells and disables iron-sulfur proteins that the cells need (Reeder et al., 2011). A study of Malassezia restricta also found that it lowers the yeast's production of lipase enzymes (Park et al., 2018). In a 1985 half-head study of 32 people, the side washed with a 1% ZPT shampoo had significantly less dandruff after three or more washes than the side washed with the same shampoo without ZPT (Marks et al., 1985).

Salicylic acid: an exfoliant

Salicylic acid does not target yeast. It loosens the outer layer of skin cells (the stratum corneum) so that scale can be shed and washed away (Arif, 2015). This is why it is used for thick, stuck-on flakes and buildup rather than for the yeast itself.

How they are regulated

In the US, both are permitted over-the-counter dandruff ingredients: pyrithione zinc at 0.3–2% in rinse-off products and salicylic acid at 1.8–3% (21 CFR 358.710). US labels for these shampoos say "For best results use at least twice a week or as directed by a doctor" (21 CFR 358.750).

In the European Union, zinc pyrithione has been banned in cosmetic products, including shampoos, since 1 March 2022, after it was classified as toxic for reproduction (category 1B) (Commission Regulation (EU) 2021/1902). Anyone who is pregnant or trying to conceive can ask a pharmacist which dandruff product to use.

Salicylic acid can be absorbed through the skin, and salicylate toxicity has been described (Arif, 2015). Ask a pharmacist first before using it on a child, over large areas of skin, or if you are allergic to aspirin or other salicylates.

Hair loss

Neither ingredient is an approved treatment for hair loss. If flaking, itching or redness does not improve with an over-the-counter shampoo, or if it comes with hair loss, see a doctor or dermatologist.

References

Arif, T. (2015). Salicylic acid as a peeling agent: A comprehensive review. Clinical, Cosmetic and Investigational Dermatology, 8, 455–461. https://pubmed.ncbi.nlm.nih.gov/26347269/

Marks, R., Pearse, A. D., & Walker, A. P. (1985). The effects of a shampoo containing zinc pyrithione on the control of dandruff. British Journal of Dermatology, 112(4), 415–422. https://pubmed.ncbi.nlm.nih.gov/3158327/

Park, M., Cho, Y.-J., Lee, Y. W., & Jung, W. H. (2018). Understanding the mechanism of action of the anti-dandruff agent zinc pyrithione against Malassezia restricta. Scientific Reports, 8, Article 12086. https://doi.org/10.1038/s41598-018-30588-2

Reeder, N. L., Kaplan, J., Xu, J., et al. (2011). Zinc pyrithione inhibits yeast growth through copper influx and inactivation of iron-sulfur proteins. Antimicrobial Agents and Chemotherapy, 55(12), 5753–5760. https://pubmed.ncbi.nlm.nih.gov/21947398/

Commission Regulation (EU) 2021/1902 of 29 October 2021 amending Annexes II, III and V to Regulation (EC) No 1223/2009 as regards the use in cosmetic products of certain substances classified as carcinogenic, mutagenic or toxic for reproduction. Official Journal of the European Union, L 387, 120. https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32021R1902

U.S. Code of Federal Regulations, 21 CFR 358.710. Active ingredients for the control of dandruff, seborrheic dermatitis, or psoriasis. https://www.ecfr.gov/current/title-21/part-358/section-358.710

U.S. Code of Federal Regulations, 21 CFR 358.750. Labeling of drug products for the control of dandruff, seborrheic dermatitis, or psoriasis. https://www.ecfr.gov/current/title-21/part-358/section-358.750