The unique therapeutic indications for PQ and the absence of alternatives have made it indispensable in the treatment of P. vivax malaria, despite its known haemotoxicity in individuals with genetic deficiency in G6PD. The efficacy and the haemotoxicity of PQ, as well as other 8-AQs, appears to be related to active metabolites, although the specific pathways and metabolites responsible, as well as their sites of formation and mechanisms of action, remain unclear.