But it has been hypothesized that primaquine (and perhaps also the newer, related drug tafenoquine) might kill a proportion of non-circulating merozoites as well as hypnozoites, such as merozoites in bone marrow, thereby reducing the number of recrudescences (not only hypnozoite-mediated relapses) that take place. Clarity in this regard is expected to be forthcoming soon. Use of primaquine in combination with quinine or chloroquine each of which is very effective at clearing P. vivax from blood, improves outcomes; they appear to also potentiate the action of primaquine.As of 2016, the US Centers for Disease Control and Prevention recommends the use of primaquine for primary prophylaxis prior to travel to areas with a high incidence of P. vivax, and for terminal prophylaxis (anti-relapse therapy) after travel.A single dose of primaquine has rapid and potent ability to kill gametocytes (stage V) of P. falciparum and P. vivax in blood; it also kills asexual trophozoites of  P. vivax in blood, but not of P. falciparum. Because of its action against gametocytes, the WHO recommends it for use in reducing transmission to control P. falciparum infections. Pneumocystis pneumonia
Primaquine is also used in the treatment of Pneumocystis pneumonia (PCP), a fungal infection commonly occurring in people with AIDS and, more rarely, in those taking immunosuppressive drugs. To treat PCP effectively, it is usually combined with clindamycin.
Special populations
Primaquine has not been studied extensively in people 65 and older so it is not known if dosing should be adjusted for this population.Primaquine should not be administered to anyone with G6PD deficiency because a severe reaction can occur, resulting in hemolytic anemia. However, the WHO has recommended that a single dose of primaquine (0.25 mg/kg) is safe to give even in individuals with G6PD deficiency, for the purpose of preventing transmission of P. falciparum malaria.Primaquine is contraindicated in pregnancy, because the glucose-6-phosphate dehydrogenase status of the fetus would be unknown.Primaquine overdose can cause a dangerous reduction in various blood cell counts, and therefore should be avoided in people at risk for agranulocytosis, which include people with conditions such as rheumatoid arthritis and lupus erythematosus, and those taking concurrent medications that also decrease blood cell counts.Hemolytic reactions (moderate to severe) may occur in individuals with G6PD deficiency and in individuals with a family or personal history of favism. Areas of high prevalence of G6PD deficiency are Africa, Southern Europe, Mediterranean region, Middle East, South-East Asia, and Oceania. People from these regions have a greater tendency to develop hemolytic anemia (due to a congenital deficiency of erythrocytic G6PD) while receiving primaquine and related drugs.