Zinc oxide (ZnO) appears as a promising preservative for pharmaceutical or cosmetic formulations. The
other ingredients of the formulations may have specific interactions with ZnO that alter its antimicrobial
properties. The influence of common formulation excipients on the antimicrobial efficacy of ZnO has
been investigated in simple model systems and in typical topical products containing a complex
formulation. A wide variety of formulation excipients have been investigated for their interactions with
ZnO: antioxidants, chelating agents, electrolytes, titanium dioxide pigment. The antimicrobial activity of
ZnO against Escherichia coli was partially inhibited by NaCl and MgSO4 salts. A synergistic influence of
uncoated titanium dioxide has been observed. The interference effects of antioxidants and chelating
agents were quite specific. The interactions of these substances with ZnO particles and with the soluble
species released by ZnO were discussed so as to reach scientific guidelines for the choice of the
ingredients. The preservative efficacy of ZnO was assessed by challenge testing in three different
formulations: an oil-in-water emulsion; awater-in-oil emulsion and a dry powder. The addition of ZnO in
complex formulations significantly improved the microbiological quality of the products, in spite of the
presence of other ingredients that modulate the antimicrobial activity.