- severity criteria: fever> 38ºc, or o2 saturation <93%. - patients with sars-cov-2 pneumonia in the acute / subacute phase. - patients who have required hospital admission for sars-cov-2. - chronic obstructive pulmonary disease (copd), asthma, bronchiectasis, pulmonary fibrosis, obstructive sleep apnea syndrome (osas), chronic respiratory failure from any cause, home oxygen therapy. - use of montelukast or zafirlukast ≤ 30 days prior to inclusion - use of any dose of systemic corticosteroids ≤ 30 days prior to inclusion - use of gemfibrocil. - hypersensitivity to montelukast or to any of the excipients included (e.g. lactose. - any condition (including the inability to swallow pills) that, in the opinion of the researcher, would prevent the completion of taking the medication. - active malignancy, current or recent chemotherapy treatment (<6 months). - medical history of infection by the human immunodeficiency virus (hiv) or any serious immunocompromised state. - pregnancy, planning to get pregnant or patients of childbearing age not undergoing birth control methods. - breastfeeding mother. - any other condition for which, in the opinion of the principal investigator, it is considered that the subject does not fit the study.

- severity criteria: fever> 38ºc, or o2 saturation <93%. - patients with sars-cov-2 pneumonia in the acute / subacute phase. - patients who have required hospital admission for sars-cov-2. - chronic obstructive pulmonary disease (copd), asthma, bronchiectasis, pulmonary fibrosis, obstructive sleep apnea syndrome (osas), chronic respiratory failure from any cause, home oxygen therapy. - use of montelukast or zafirlukast ≤ 30 days prior to inclusion - use of any dose of systemic corticosteroids ≤ 30 days prior to inclusion - use of gemfibrocil. - hypersensitivity to montelukast or to any of the excipients included (e.g. lactose. - any condition (including the inability to swallow pills) that, in the opinion of the researcher, would prevent the completion of taking the medication. - active malignancy, current or recent chemotherapy treatment (<6 months). - medical history of infection by the human immunodeficiency virus (hiv) or any serious immunocompromised state. - pregnancy, planning to get pregnant or patients of childbearing age not undergoing birth control methods. - breastfeeding mother. - any other condition for which, in the opinion of the principal investigator, it is considered that the subject does not fit the study.