Chloroquine and primaquine, has been first therapy for Plasmodium vivax and ovale malaria for over 50 years, and has been part of National policy in Thailand for decades. Primaquine is the only available hypnozoitocidal drug for P.vivax and gametocytocidal agent for Plasmodium falciparum malaria. Despite this enormous use knowledge about the mechanism of activity, pharmacokinetic properties, resistance and toxicity of primaquine are limited. Primaquine at the previously used dose for radical cure (15 mg base/day for 14 days) is also weakly effective against asexual stages of P. vivax malaria [4] The higher dose of 30 mg daily evaluated for malaria prophylaxis is now also generally recommended for radical cure. In the standard radical cure regimen, primaquine is usually given after the 3-day course of chloroquine, but there has been no study to inform timing of dosing so this decision is arbitrary. Despite the multistage specificity against the malaria parasite, and extensive recommendations, primaquine is currently underused because of uncertainties over safety, efficacy and dosage. Primaquine is an oxidant drug and causes haemolysis in patients with glucose-6-phosphate dehydrogenase deficiency (G6PD). Although several studies of the metabolites of the primaquine and chloroquine have been conducted over the past few years remarkably little is known about the interaction between these two commonly coadministered drugs.