Primaquine is the only widely available drug to prevent relapses of Plasmodium vivax malaria. Primaquine is underused because of concerns over oxidant hemolysis in glucose-6-phosphate dehydrogenase (G6PD) deficiency. A pharmacometric trial showed that ascending-dose radical cure primaquine regimens causing 'slow burn' hemolysis were safe in G6PD-deficient Thai and Burmese male volunteers. We developed and calibrated a within-host model of primaquine hemolysis in G6PD deficiency, using detailed serial hemoglobin and reticulocyte count data from 23 hemizygote deficient volunteers given ascending-dose primaquine (1,523 individual measurements over 656 unique time points). We estimate that primaquine doses of ~0.75 mg base/kg reduce the circulating lifespan of deficient erythrocytes by ~30 days in individuals with common Southeast Asian G6PD variants. We predict that 5 mg/kg primaquine total dose can be administered safely to G6PD-deficient individuals over 14 days with expected hemoglobin drops of 18 to 43% (2.7 to 6.5 g/dL drop from a baseline of 15 g/dL).CLINICAL TRIALSThis study is registered with the Thai Clinical Trials Registry (TCTR) as TCTR20170830002 and TCTR20220317004.