To improve the quality of life of the growing number of patients with cancer, it is important to manage the adverse effects that occur during their treatment. Nausea and vomiting are the most common adverse effects of radiation therapy and chemotherapy; they result from therapy-associated damage to the inner cells of the digestive system [1]. Their persistence can cause appetite loss, weight loss, and dehydration, subsequently leading to a loss of nutrition and energy. Consequently, this delays the treatment and recovery of patients with cancer [2,3]. Therefore, nausea and vomiting are critical factors for successful cancer treatment, and their management is essential.

While nausea and vomiting can arise from surgical procedures or radiotherapy, chemotherapy-induced nausea and vomiting (CINV) is often the most severe and distressing manifestation. Both peripheral and central nervous system pathways contribute to the pathophysiology of acute and delayed CINV. In acute CINV, toxic chemotherapeutic agents induce free radical formation, which triggers a serotonin release; serotonin binds to the intestinal vagal afferent nerves and further triggers the vomiting reflex. In delayed CINV, Substance P assumes prominence as the primary neurotransmitter and induces vomiting by binding to the neurokinin-1 receptors within the nucleus of the solitary tract [4,5].

Drugs have been used to manage nausea and vomiting in patients undergoing treatment for cancer; commonly used drugs include serotonin receptor antagonists (e.g., 5-hydroxytryptamine [5-HT3] receptor antagonists), corticosteroids, and antiemetics (e.g., neurokinin-1 receptor antagonists). However, nausea and vomiting are known to persist despite treatment [6]. Furthermore, antiemetics can further cause adverse effects (such as anxiety, depression, headache, hypotension, constipation, and diarrhea) due to their interactions with chemotherapeutic agents and other drugs [7].

Although non-pharmacological methods may have a lower efficacy than pharmacological methods, their advantages include fewer adverse effects and lower costs; furthermore, they may exert greater efficacy when used in combination with antiemetics. Thus, non-pharmacological methods may be appropriately combined with pharmacological treatments in accordance with each patient's condition and preferences [8]. Among non-pharmacological methods, aromatherapy is widely used in patients with cancer because it is relatively easier to access [9]. Aromatherapy helps reduce physical stress by stimulating the nervous system, respiratory system, and circulation through fragrance. The essential oils used in aromatherapy are known to stabilize brain waves and reduce stress-induced nausea and vomiting [10,11]. Furthermore, these oils harmonize with the receptor cells in the nasal passage during inhalation. This initiates a transmission to the limbic system and hypothalamus, facilitated by the olfactory bulb. These signals then prompt the brain to release neurotransmitters, such as serotonin and endorphins. These bioactive compounds establish a connection between the nervous system and the body, engendering a profound sense of relief [12,13].

Existing meta-analyses have only examined the effect of inhalation aromatherapy on CINV [11,14], and concerns exist about expanding the interpretation by combining findings from randomized controlled trials (RCTs) and non-RCTs. In addition, practical applications are limited by a lack of analyses involving specific oils and intervention methods. To date, no studies have reported reliable evidence on the effect of aromatherapy on nausea and vomiting in patients with cancer.

Therefore, this study aimed to investigate the effect of aromatherapy in reducing nausea and vomiting in patients with cancer according to the delivery method, essential oil type, and intervention length; our findings may provide a valid basis for the establishment of practical aromatherapy guidelines for this population.