We report the clinical outcomes following implementation of initial COVID-19 treatment guidelines in Thailand. A composite poor outcome was defined as death, ICU admission, requiring intubation, or high-flow oxygen. 744 COVID-19 patients (48.8% male) were included, median (IQR) age was 37 (27-48) years [8.4% >60 years] and 21.4% had pneumonia at admission. Admission <4 days from symptom onset had a reduced risk of poor outcome. In a subgroup analysis, favipiravir use reduced the risk of a poor outcome for patients admitted <4 days from symptom onset (OR 0.320 (0.152-0.662), P=0.003). Thai guidelines now include favipiravir to treat all symptomatic COVID-19 patients.