**Abstract**

Essential oils are widely used in the pharmaceutical industry for their antimicrobial, antiviral, antifungal, antiparasitic, and insecticidal properties. Their anticancer activity has been increasingly explored as the natural constituents of essential oils play an important role in cancer prevention and treatment. The chemical composition of essential oils includes monoterpenes, sesquiterpenes, oxygenated monoterpenes, phenolic sesquiterpenes, and others. Several mechanisms of action such as antioxidant, antimutagenic, antiproliferative, enhancement of immune functions, modulation of multidrug resistance, and synergistic mechanism of volatile constituents are responsible for their chemotherapeutic properties. This review focuses on the activity of essential oils and their chemical composition in regard to breast cancer.

**Keywords:** essential oils, antitumor activity, chemical composition, antitumor mechanism, breast cancer

## **1. Introduction**

Cancer is a disease in which normal cells change into a type of cell that can continuously proliferate and, by a process named metastasis, migrate to distant parts of the body [1]. Breast cancer (BC) is the most common cancer in women in the world, presenting high morbidity and mortality [2]. It causes a major public health problem, and the incidence is increasing all over the world [3].

The treatment used for cancer causes many side effects; besides, there are a large number of cases of resistance toward anticancer drugs [4]. These are the main causes that limit the success of treatment in aggressive BC cases. Thus, the need to have novel therapeutic agents is urgent [2].

Natural products, such as plants, may hold the future of BC treatment as the source for new drugs that can interfere with certain processes and ultimately result in clinical usage as an adjuvant therapy [1].

Essential oils (EOs) act as protective mechanism for plants against bacteria, viruses, insects, and even herbivores [5]. They are widely used by the population to treat cancer and can change the metabolism of cancer cells in very low doses, besides provide energy for synthetic processes [6]. This way, EOs are being considered as a promising agent opening venues for novel anticancer therapy as a way to defeat side

effects and the high cost of chemotherapy approaches in BC [7]. This review focuses on apoptosis as an action mechanism by EOs in breast cancer cells, antitumoral activity of EOs and their bioactive compounds, and optimization of EOs' use and their potential as an alternative for side effects reduction during breast cancer treatment.
