Tafenoquine was recently approved by regulatory authorities in the USA and Australia. Tafenoquine is an alternative radical curative treatment to primaquine acting against the dormant liver stage of Plasmodium vivax (the hypnozoite). Tafenoquine (an 8-aminoquinoline) has the substantial advantage of single dosing as compared to a 14-day course of primaquine to achieve radical cure. The recommended tafenoquine dose is 300 mg, which was shown to be significantly worse in radical curative efficacy to a total primaquine dose of 3.5 mg/kg in Southeast Asia. The cure rate of tafenoquine 300 mg in Southeast Asian study sites was only 74%. The comparator 3.5 mg/kg total primaquine dose is the standard and most commonly used dose globally, but in Southeast Asia and the Western Pacific, higher doses of primaquine are needed for radical cure. This study aims to determine the optimal dose of tafenoquine in Southeast Asia.