This study was a clinical trial with a two-group pretest-posttest design and three blinders, in which the main researcher, patients (due to reduced level of consciousness), evaluator and statistical analyzer were unaware of the study groups.

This study was designed as an equivalence trial to compare the effects of licorice and chlorhexidine mouthwash on the oral health of mechanically ventilated patients in the intensive care unit. The aim was to demonstrate that licorice mouthwash is not inferior to chlorhexidine mouthwash and could be a suitable alternative. The equivalence margin was determined based on clinical judgment and statistical considerations, and the sample size was calculated to ensure sufficient power to confirm the hypothesis of equivalence.

This study involved the participation of intubated patients admitted to the Qaem ICU of Shahid Beheshti Hospital in Hamadan, located in northwest Iran, from February 2022 to September 2023. Study inclusion required individuals aged 18 to 65 with an oral endotracheal tube, airway, or nasogastric tube, without coagulation and immune disorders, pulmonary infections, sepsis, allergies to study substances, pregnancy, or prior antibiotic use. Exclusion applied to those with hypokalemia, smoking history, coronary artery disease, heart failure, renal failure, or hypertension. Exclusion criteria included the death or transfer of patients, premature removal of the endotracheal tube, withdrawal from the study by the individual or legal guardian, or the occurrence of complications from licorice consumption such as renal failure, hypertension, hypokalemia, and cardiac dysrhythmia [20]. Considering a confidence level of 95% and a power of 80%, a mean difference of 0.5, a variance of 0.93 and a variance of 0.92 [19], and a 20% probability of sample dropout, the sample size in each group was estimated to be at least 65 persons.

First, patients who met the inclusion criteria were selected by convenience sampling, and then, sequentially numbered, sealed, opaque envelopes were used. Patients were allocated to two groups: group A (0.2% chlorhexidine digluconate group) and group B (20% liquorice group). In each of the chlorhexidine and liquorice mouthwash groups, 65 patients entered the study, but 5 patients from each group dropped out; thus, 60 patients completed the study (Fig. 1).

Study flow diagram based on the Consort statement of the year 2010

The data collection tools included a demographic and clinical characteristics questionnaire of patients and the Beck Oral Assessment Scale (BOAS).

The demographic and clinical characteristics of the patients included age, sex, body mass index, method of nutrition, GCS score, FOUR score, cause of hospitalization, underlying disease, number of teeth, blood pressure and heart rate.

We used the Beck Oral Assessment Scale (BOAS) to assess oral health. This scale has five subscales: lips, mucosa and gums, teeth, tongue and saliva. Each subscale is graded from one to four based on health status. The total score ranges from five to 20, where five indicates no disorder, six to 10 indicates mild disorder, 11 to 15 indicates moderate disorder and 16 to 20 indicates severe disorder [21, 22]. This scale was translated and validated by Safarabadi et al. (2012) in Iran [21]. They assessed and confirmed its validity using face validity, content validity and construct validity. They also confirmed its reliability using the interrater reliability method with a kappa coefficient of 0.92 [22]. In this study, the kappa coefficient for each dimension was estimated to range from 0.88 to 0.94, and the kappa coefficient of the total questionnaire was 0.91.

In Group A, a 0.2% chlorhexidine digluconate mouthwash produced by Shahre Daru Company (http://www.shahredaru.com/en/contact) in Tehran, Iran, was employed. In contrast, Group B used a 20% liquorice mouthwash solution. This solution was formulated to achieve the minimum inhibitory and bactericidal concentrations required to target periodontal pathogens, resulting in a 20% (w/v) aqueous extract of licorice root. Mouthwash chlorhexidine digluconate was added by a pharmacist to a container labeled A, and the mouthwash liquorice was added to another container labeled B and was given to the main researcher; until the end of the study, only the pharmacist knew about their contents.

Before the intervention, one of the morning shift nurses (nursing expert) who was unaware of the study groups completed the demographic and clinical characteristics questionnaire and the Beck Oral Assessment Scale for both groups. The nurse used information from the file and assessed the oral health and teeth of the participants using a flashlight and a special mirror for dentistry. The nurse received the necessary training under the supervision of an oral health specialist. In both groups, before performing oral care, the cuff pressure was adjusted to between 25 and 20 mmHg using a special manometer to ensure its appropriateness. The headboard was raised to 30 degrees, and the patient’s head was turned to one side. A towel was spread around the chin and head of the patient. A receiver was placed next to the patient’s mouth. Twice a day (morning and evening), a special soft toothbrush for children was used to brush all areas of the mouth, including the internal and external surfaces of the teeth (with circular movements), the gums and the tongue (with movements from back to front), without using toothpaste. After brushing, the oral cavity was rinsed with normal saline and suctioned. During each brushing, the airway of the samples under study was removed, cleaned and placed back in the patient’s mouth after using mouthwash. All these procedures in both groups were the same and were performed by the main researcher.

Intervention group A: In group A, in addition to the intervention described above, all parts of the mouth were rinsed with a syringe containing 15 ml of mouthwash solution with label A (chlorhexidine digluconate 0.2%) immediately after each brushing (twice a day at 12-hour intervals) and suctioned after 30 s. The intervention lasted for 5.5 days. Intervention group B: In group B, the interventions were the same as those in group A, except that mouthwash with label B (20% licorice mouthwash) was used instead of mouthwash A. Finally, on the morning of the sixth day, after the oral care of the patients by the main researcher following the research protocol, oral health was assessed and recorded by the same nurse who used the Beck scale before the intervention.

The data were analyzed using SPSS software version 16, and descriptive and inferential statistics, including the Kolmogorov‒Smirnov test (to check the normality of the data distribution), chi‒square tests and Fisher’s exact tests (to compare categorical variables), independent t tests (to compare continuous variables between two groups) and paired t tests (to compare continuous variables within each group), were used. The significance level was set at less than 0.05.