Table 1 Inclusion and exclusion criteria for the study participants

Inclusion criteria: participants are required to meet all inclusion criteria.

 a) Individuals ≥ 18 and ≤ 80 years old with SARS-CoV-2 infection (positive SARS-CoV-2 detection test (RT-PCR), antigenic test or equivalent < 10 days from the onset of symptoms) treated in primary care.

 b) Persistent respiratory symptoms lasting for more than 4 weeks and less than 1 year.

 c) Mild to moderate dyspnea: score from 1 to 3 at the beginning of the study according to the Modified Medical Research Council (mMRC) scale.

 d) Patient must sign an informed consent form.

Exclusion criteria: participants meeting one or more of the following criteria will be excluded.

 a) Severity criteria: fever> 38 °C, O₂ saturation < 93%.

 b) Patients with pneumonia in the acute/subacute phase due to SARS-Cov-2.

 c) Patients who have required hospital admission for SARS-Cov-2.

 d) Chronic respiratory disease: chronic obstructive pulmonary disease (COPD), asthma, bronchiectasis, pulmonary fibrosis, obstructive sleep apnea syndrome (OSAS), chronic respiratory failure from any cause, and home oxygen therapy.

 e) Use of montelukast or zafirlukast ≤ 30 days prior inclusion.

 f) Use of gemfibrozil.

 g) Hypersensitivity to montelukast, lactose intolerance, or any of the excipients of study treatment or placebo.

 h) Active malignancy and current or recent chemotherapy treatment (< 6 months).

 i) Medical history of human immunodeficiency virus (HIV) infection or any severe immunosuppression.

 j) Patients who have been in a clinical trial 30 days prior to the study.

 k) Pregnancy or planning a pregnancy.

 l) Breastfeeding mother.

 m) The principal investigator considers that the subject will not be able to perform the test procedures.