Study aim: Evaluation of safety and effectiveness of doxycycline in the treatment of hospitalized patients with Covid-19 Design: Open label phase 3 clinical trial with a control group and parallel groups on 180 randomized patients; Random Allocation Software was used for randomization. Settings and conduct: This study will be performed on patients with Covid 19 in Firoozgar Hospital in Tehran and the efficacy of dixycycline in them will be evaluated. Patients in both groups are treated with the standard protocol or dioxycycline, this study is not blinded and the necessary information will be collected using a checklist. Participants/Inclusion and exclusion criteria: Inclusion criteria include age more than 18 years, definitive diagnosis of Covid 19 and having informed consent. Also, exclusion criteria included allergies to dioxycycline, chronic liver failure, chronic renal failure, history of lupus, history of porphyria, pregnancy or breastfeeding. Intervention groups: Control group: Standard protocol will be used for ten days including daily oxygen with the rate of 5 to 7 liters per minute, observance of hygienic principles and subcutaneous use of heparin manufactured by Caspian Pharmaceutical Company 5000 units three times per day, for 10 days. Intervention group: In addition to the national standard treatment protocol, doxycycline 200 mg capsules made by Pars Behroozan Company are used initially and then doxycycline 100 mg capsules twice a day for 7 days. Main outcome variables: need for ICU admission and duration of hospitalization