A 50-year-old British expat living in Lagos, Nigeria, develops P. falciparum malaria with a parasitaemia of 15%, prostration, and acidotic breathing (pH 7.22, bicarbonate 12 mmol/L, lactate 8.2 mmol/L). He receives IV artesunate. At 24 hours, parasitaemia is 11%. At 48 hours, parasitaemia remains at 9%. What is the most important consideration?