Introduction Favipiravir is an antiviral agent that is recently used for SARS-CoV2 infection. The drug-drug interactions of favipiravir especially with chemotherapeutic agents in a patient with malignancy are not well known. Case report The patient diagnosed with metastatic osteosarcoma was given high dose methotrexate treatment, and favipiravir was started on the third day of the treatment with suspicion of SARS-CoV2 infection. Grade 3 hepatotoxicity developed after favipiravir. Management & outcome: The acute viral hepatitis panel and autoimmune liver disease panel were negative. The ultrasound of the abdomen was unremarkable for any hepatobiliary pathology. The all viral and hepatobiliary possible etiological factors were ruled out. The patient’s liver enzymes increased just after (12 hours later) the initiation of favipiravir, and we diagnosed toxic hepatitis caused by favipiravir-methotrexate interaction. Therefore, methylprednisolone 1 mg/kg dose was started for a presumed diagnosis of toxic hepatitis. Hepatotoxicity completely regressed after favipiravir was discontinued. Discussion Favipiravir may inhibit methotrexate elimination by inhibiting aldehyde oxidase and its sequential use may cause hepatotoxicity in this case. The clinicians should keep in mind possible drug interactions while using new antiviral agents against SARS-CoV2 like favipiravir.