Study aim: Determining the effect of Montelukast on the prevention of nasal polyp recurrence after endoscopic functional sinus surgery Design: Clinical trial with an intervention group, phase 3, sample size 69, with a parallel group; One side blind, randomized using block randomization Settings and conduct: This study is conducted in Razi Birjand Hospital, South Khorasan Province. 138 patients were randomly divided into two intervention groups, one or two (A-B). Questions related to the effectiveness of the intervention before and 6 weeks after the treatment will be asked and evaluated by the moderator. Participants/Inclusion and exclusion criteria: Patients with chronic rhinosinusitis will be referred to the Otolaryngology Clinic of Razi Birjand Hospital in 2023. The inclusion criteria include: age over 18 years, taking medication regularly, consent to participate in the study, cooperation with the present research team, candidate for endoscopic sinus surgery. Exclusion criteria include: patients with exacerbation of recurrent acute rhinosinusitis, rhinosinusitis without polyposis, sinusitis associated with ciliary dyskinesia or sinusitis associated with cystic fibrosis. Intervention groups: The intervention groups are as follows: Group A receiving Montelukast 10 mg once a day at night and Group B receiving a topical steroid such as fluticasone 0.1 mg twice a day, 50 micrograms each time. Main outcome variables: Runny nose, improvement of symptoms compared to before taking the drug, clinical symptoms include: headache, nasal congestion, sinus pain, PND, pain around the eyes, pus in the nose, cough, runny nose and decreased sense of smell