## 1 Introduction  
With the increasing interest in natural and organic cosmetics, both manufacturers and consumers are looking for solutions that not only minimize the risk of allergies or irritations but also effectively protect cosmetic products from microbiological spoilage. Many cosme contain conventional preservatives such as parabens, phenoxyethanol, or formaldehyde, which effectively eliminate or inhibit the growth of microorganisms. However, numerous reports regarding the potential side effects of these substances have led many people to avoid cosmetics containing them, fearing the possibility of contact allergies and their impact on hormonal balance [1; 2]. Additionally, the long-term use of these preservatives may lead to the development of microbial resistance, which further motivates the search for alternatives [3]. In this context, natural compounds such as eugenol and isoeugenol appear to be promising alternatives. Eugenol is a phenolic monoterpene compound found in clove oil (_Syzygium aromaticum_), allspice oil, and basil oil, as well as in some plants belonging to the Myrtaceae and Lamiaceae families [1; 2] (Figure 1). Isoeugenol, structurally similar to eugenol, is a compound found in the iris root oil. Both compounds exhibit strong antibacterial, antifungal, and antioxidant properties, which can positively influence the stability of cosmetic formulations by protecting them from microbiological and oxidative degradation [3]. Eugenol and isoeugenol interact with the cell membranes of bacteria and microorganisms, disrupting their integrity and leading to the loss of essential ions and nutrients for the microorganisms. These compounds also exhibit the ability to inhibit the cellular enzymes, which are responsible for the metabolic processes of pathogens, which can lead to their neutralization or growth inhibition. In vitro studies showed that eugenol and isoeugenol are active compounds against a wide spectrum of both Gram-positive and Gram-negative bacteria, including _Staphylococcus aureus_, _Escherichia coli_, _Pseudomonas aeruginosa_, and various species of fungi [4]. Their action primarily involves interaction with the phospholipids in the cell membrane, increasing the membrane permeability, and leading to the leakage of cellular substances such as proteins and ions, ultimately resulting in microbial death [5; 6]. The antioxidant properties of eugenol and isoeugenol also contribute to the stability of cosmetics, as they prevent the oxidation of lipids and other cosmetic ingredients that  
Figure 1: Structure of eugenol and isoeugenol. may decompose due to exposure to oxygen or light. Many studies showed that eugenol acts as a free radical scavenger, protecting cells from oxidative stress and contributing to the long-term stability of cosmetic ingredients [4; 5; 6]. Due to their chemical properties, eugenol and isoeugenol can play the function of preservatives in cosmetics without the need for synthetic substances. It should be emphasized that they exhibit effectiveness at lower concentrations than many synthetic preservatives and it reduces the risk of potential irritation and allergies. Additionally, their use as preservatives may limit the development of microbial resistance, as their multifaceted mechanism of action (antibacterial and antifungal) makes it difficult for bacteria to develop resistance [5]. However, it is important to note that despite their natural origin, the use of eugenol and isoeugenol also requires controlled studies regarding their potential impact on the skin, especially during prolonged application [4; 5]. The existing studies indicate a favorable safety profile for these substances; however, their irritating properties at higher concentrations require special attention during the preparation of formulations of cosmetics [6]. The growing \"clean beauty\" trend, which emphasizes products free from harmful or suspicious synthetic substances, perfectly aligns with the concept of using eugenol and isoeugenol as preservatives and antibacterial agents.