Primaquine is a decades-old antimalarial drug with a unique ability: it can kill dormant malaria parasites hiding inside liver cells, something most other antimalarials cannot do. First synthesized in 1946, it remains the most widely used drug for preventing relapses of Plasmodium vivax and P. ovale malaria and for blocking transmission of P. falciparum.1PubMed. Primaquine revisited six decades after its discovery But primaquine carries a serious risk for people with glucose-6-phosphate dehydrogenase (G6PD) deficiency, an inherited enzyme shortage common in the very regions where malaria is endemic. That tension between a drug the world needs and a side effect that can be life-threatening shapes nearly every clinical decision around primaquine.
What Primaquine Actually Does
Most antimalarials target parasites circulating in the bloodstream. Primaquine’s value lies in reaching stages of the malaria parasite that blood-stage drugs miss. For P. vivax and P. ovale, the parasite forms dormant stages called hypnozoites in the liver. These can reactivate weeks, months, or even years after the initial infection, causing a full relapse. Primaquine is one of only two drugs licensed to clear hypnozoites, a process called radical cure.2PubMed Central. Primaquine for Plasmodium vivax radical cure: What we do not know and why it matters Animal studies suggest it does not instantly destroy hypnozoites but instead arrests developing liver-stage parasites and blocks hypnozoite reactivation.3Cell Host & Microbe. In Vivo Model of Plasmodium vivax Hypnozoite Latency, Activation, and Drug Efficacy
Primaquine also has a separate role in P. falciparum malaria. It kills gametocytes, the sexual stage of the parasite that mosquitoes pick up during a blood meal. A single dose added to standard treatment can help stop onward transmission, which is especially useful in areas fighting artemisinin resistance.4PubMed. Primaquine to prevent transmission of falciparum malaria No other widely available antimalarial has this transmission-blocking property to the same degree.
Side Effects Beyond Hemolysis
The side effect that gets the most attention is hemolytic anemia in G6PD-deficient individuals, covered in detail below. But primaquine has other side effects that affect everyone to some extent.
Gastrointestinal symptoms are the most common complaint. A large individual-patient-data meta-analysis found that stomach problems increased with dose: roughly 4% of patients who received no primaquine reported gastrointestinal issues, compared with about 11% of those given a high daily dose.5The Lancet Infectious Diseases. Efficacy and tolerability of primaquine regimens for the radical cure of Plasmodium vivax malaria: a systematic review and individual patient data meta-analysis Taking the pills with food usually helps, but nausea, abdominal cramping, and vomiting are reasons some people abandon the course early.
Methemoglobinemia is another known effect. Primaquine’s metabolites oxidize hemoglobin into methemoglobin, which cannot carry oxygen efficiently. In a study of 54 patients on a standard 14-day course, methemoglobin levels ranged from about 3% to 7%, which is mildly elevated but rarely produces symptoms in otherwise healthy people.6PubMed Central. Primaquine in Plasma and Methemoglobinemia in Patients with Malaria Due to Plasmodium vivax in the Brazilian Amazon Basin In rare cases, especially at higher doses or in people with other hemoglobin disorders, methemoglobin levels can climb high enough to cause shortness of breath and blue-tinged skin. The metabolites responsible appear to be different from primaquine itself, since researchers found no clear correlation between the drug’s blood concentration and methemoglobin levels.
Why G6PD Deficiency Makes Primaquine Dangerous
G6PD is an enzyme that protects red blood cells from oxidative damage. It is the first step in a biochemical pathway that keeps a steady supply of a molecule called NADPH, which red blood cells rely on to neutralize reactive oxygen species.7PubMed. Glucose-6-phosphate dehydrogenase–from oxidative stress to cellular functions and degenerative diseases When G6PD activity is low, red blood cells lose their main antioxidant defense and become vulnerable to drugs that generate oxidative stress.
Primaquine’s active metabolites, particularly 5-hydroxyprimaquine, do exactly that: they flood red blood cells with oxidative compounds.8PubMed Central. Molecular Mechanisms of Drug-Induced Hemolysis in G6PD Deficiency: Mechanistic Insights In people with enough G6PD, the cells cope. In people with deficient G6PD, the red blood cells rupture en masse, causing hemolytic anemia. Symptoms include dark urine, fatigue, rapid heart rate, and in severe cases, kidney injury or the need for blood transfusion.
The cruelty of this interaction is geographic. G6PD deficiency is most common in populations with a long history of malaria exposure, because having lower G6PD activity appears to offer some protection against malaria parasites. So the people most likely to need primaquine are also the people most likely to be harmed by it.9PubMed. G6PD deficiency: global distribution, genetic variants and primaquine therapy The deficiency is X-linked, meaning men with one copy of a deficient gene are fully affected, while women can be carriers with a range of enzyme activity from near-normal to severely low.
The Problem with Screening Women
Standard G6PD screening tests, especially the older qualitative ones that give a simple normal/deficient result, work reasonably well for men. But they frequently miss women who are heterozygous carriers. Because G6PD is on the X chromosome and women have two X chromosomes, random inactivation of one X in each cell means a heterozygous woman can have a mosaic of normal and deficient red blood cells. Some of her cells produce plenty of G6PD; others produce almost none. If enough of her cells happen to express the normal copy, she will test “normal” on a qualitative screening test, even though a large fraction of her red blood cells are vulnerable to primaquine-induced hemolysis.10PLoS Medicine. Haemolysis in G6PD Heterozygous Females Treated with Primaquine for Plasmodium vivax Malaria: A Nested Cohort in a Trial of Radical Curative Regimens
This is not a theoretical risk. Heterozygous women who test normal but have intermediate G6PD activity can and do develop clinically significant hemolysis on primaquine. Recognizing this gap has pushed the field toward quantitative point-of-care tests that measure actual enzyme activity rather than giving a binary yes/no answer.
How G6PD Testing Works Today
The move toward quantitative testing has produced devices like the STANDARD G6PD Biosensor (SD Biosensor), which measures enzyme activity from a finger-prick blood sample in a few minutes. A systematic review and meta-analysis found this device had pooled sensitivity of about 99% and specificity of about 98% for detecting severe deficiency, with performance remaining strong even at higher activity thresholds used to identify intermediate-deficiency women.11PubMed Central. Diagnostic accuracy of the point-of-care standard G6PD test™ (SD Biosensor) for glucose-6-phosphate dehydrogenase deficiency: a systematic review and meta-analysis Not all point-of-care devices perform equally. An older qualitative test, the CareStart, showed notably lower sensitivity: around 82% at the severe-deficiency threshold and dropping to about 54% at the intermediate threshold, though specificity stayed high.12PubMed. Diagnostic Accuracy of Point-of-Care Devices for Glucose-6-Phosphate Dehydrogenase Deficiency Detection Among Malaria Patients: A Systematic Review and Meta-Analysis In practice, that means the CareStart misses a substantial fraction of people with intermediate deficiency, exactly the group most likely to be heterozygous women.
Despite better technology being available, many malaria-endemic countries still do not routinely test for G6PD before prescribing primaquine. A study of barriers to implementation found that health workers and policymakers in several countries questioned whether routine testing was cost-effective, especially where G6PD deficiency prevalence was perceived to be low.13PubMed Central. Barriers to routine G6PD testing prior to treatment with primaquine Economic modeling studies have tried to settle this. One analysis from Laos found that adding G6PD testing before unsupervised primaquine cost roughly $97 per disability-adjusted life year averted, well below standard cost-effectiveness thresholds.14PLOS ONE. Cost-effectiveness analysis of G6PD diagnostic test for Plasmodium vivax radical cure in Lao PDR: An economic modelling study A separate analysis from the Thailand-Myanmar border reached broadly similar conclusions, though it found the value dropped when the impact of radical cure on recurrences was assumed to be low.15PLOS Neglected Tropical Diseases. Using G6PD tests to enable the safe treatment of Plasmodium vivax infections with primaquine on the Thailand-Myanmar border: A cost-effectiveness analysis
The G6PD Variant Map
G6PD deficiency is not one uniform condition. Over 200 genetic variants have been cataloged, and they vary sharply by region. Across sub-Saharan Africa, the landscape is dominated by one variant, known as G6PD A-. In western Asia, the severe Mediterranean variant predominates. Further east in China and the Asia-Pacific, no single variant dominates, and the genetic diversity is much higher.16PubMed Central. Spatial distribution of G6PD deficiency variants across malaria-endemic regions This matters because different variants leave people with different residual enzyme activity and therefore different levels of risk from primaquine. Someone with the African A- variant typically retains more enzyme function than someone with the Mediterranean variant, who may have almost none. A one-size-fits-all dosing approach does not capture this variability well.
Why So Many People Do Not Finish the Full Course
The standard radical cure regimen is primaquine daily for 14 days alongside a blood-stage drug like chloroquine. That is a long course for a patient whose fever and symptoms typically resolve within two or three days of starting treatment. The incentive to keep taking pills that seem unnecessary, and that may cause stomach upset, drops quickly.17PLoS Medicine. Unsupervised primaquine for the treatment of Plasmodium vivax malaria relapses in southern Papua: A hospital-based cohort study One hospital-based study in Papua found that prescribing the full 14-day course was associated with only a modest reduction of about 10% in the rate of returning with vivax malaria within a year, likely because so few patients actually completed it.
A large meta-analysis of individual patient data quantified the consequences: the risk of a vivax recurrence by day 90 was about 14% in patients with poor adherence, compared with roughly 6% in those who completed their full course. After adjusting for confounders, poor adherence more than doubled the recurrence rate.18PubMed Central. Effect of adherence to primaquine on the risk of Plasmodium vivax recurrence: a WorldWide Antimalarial Resistance Network systematic review and individual patient data meta-analysis Every recurrence is not just another bout of illness for the patient; it is another opportunity for the parasite to circulate and infect mosquitoes.
Shorter Regimens and Tafenoquine
The adherence problem has driven a search for shorter radical cure courses. A large multi-country trial found that a 7-day course of higher-dose primaquine was non-inferior to the standard 14-day course, with recurrence rates of about 0.18 per person-year versus 0.16, well within the predefined margin. Without any primaquine, the rate was nearly 1 recurrence per person-year.19The Lancet. Short-course primaquine for the radical cure of Plasmodium vivax malaria: a multicentre, randomised, placebo-controlled non-inferiority trial A more recent trial (the EFFORT trial) compared the same 7-day high-dose primaquine, single-dose tafenoquine, and the standard 14-day course. Both shorter options showed a lower risk of recurrence at six months, though the difference did not quite reach statistical significance for the 7-day arm. Both were well tolerated in people with normal G6PD activity.20PubMed. Effectiveness and safety of 7-day high-dose primaquine and single-dose tafenoquine versus 14-day low-dose primaquine in patients with Plasmodium vivax malaria (EFFORT)
Tafenoquine, approved in 2018, is the most dramatic simplification: a single dose. A pivotal trial assigned patients to either one 300 mg dose of tafenoquine or the standard 14-day primaquine course, both alongside chloroquine.21PubMed Central. Tafenoquine versus Primaquine to Prevent Relapse of Plasmodium vivax Malaria Real-world implementation data from the Brazilian Amazon showed tafenoquine outperforming 7-day primaquine, with recurrence-free effectiveness of about 89% versus 84% at day 90.22The Lancet Infectious Diseases. Effectiveness of single-dose tafenoquine versus 7-day primaquine for Plasmodium vivax radical cure in the Brazilian Amazon: an observational implementation study The trade-off is that because tafenoquine’s long half-life means you cannot simply stop the drug if hemolysis starts, it requires even more stringent G6PD screening than primaquine does. G6PD testing is mandatory before tafenoquine can be given.
What Happens When G6PD-Deficient Patients Need Radical Cure
For patients confirmed to have G6PD deficiency, clinicians can use a weekly primaquine regimen instead of the daily one. The idea is that giving a dose once a week allows the body time to regenerate red blood cells between each hemolytic hit. A study in Cambodia found that about 28% of G6PD-deficient patients on this weekly schedule had a hemoglobin drop exceeding 25%, compared with none in the G6PD-normal group. One patient needed a blood transfusion, but no one developed severe anemia or kidney injury.23PubMed Central. Tolerability and safety of weekly primaquine against relapse of Plasmodium vivax in Cambodians with glucose-6-phosphate dehydrogenase deficiency A more recent study saw two of 26 G6PD-deficient men experience hemoglobin drops over 5 g/dL after the first dose, but both were able to complete the 8-week course; one patient temporarily missed doses and recovered before resuming.24PLOS Neglected Tropical Diseases. Weekly primaquine for radical cure of patients with Plasmodium vivax malaria and glucose-6-phosphate dehydrogenase deficiency This approach works but requires close monitoring, ideally with weekly hemoglobin checks, which adds logistical burden in remote settings.
When Primaquine Fails Despite Normal G6PD
Some patients relapse repeatedly even though they completed a full primaquine course and have normal G6PD activity. The explanation, in many cases, involves a different enzyme entirely: CYP2D6. Primaquine is a prodrug that the liver must convert into active metabolites, and CYP2D6 is the main enzyme responsible for that conversion.25PubMed Central. Increased primaquine total dose prevents Plasmodium vivax relapses in patients with impaired CYP2D6 activity: report of three cases People who carry genetic variants that reduce CYP2D6 activity metabolize primaquine poorly, and their relapse risk is markedly higher. A study comparing patients with low versus adequate CYP2D6 activity found that the odds of relapse were roughly 9 times higher in those with impaired genotypic activity scores, and when enzyme function was measured directly in a metabolic test, the odds climbed even further.26JAMA Network Open. Association of Impaired Cytochrome P450 2D6 Activity Genotype and Phenotype With Therapeutic Efficacy of Primaquine Treatment for Latent Plasmodium vivax Malaria
CYP2D6 variation is common. Depending on the population, anywhere from about 5% to over 10% of people are poor metabolizers. Unlike G6PD testing, CYP2D6 testing is not yet standard practice before primaquine is prescribed, partly because it requires genotyping that is not available in most malaria-endemic settings. A case report illustrates the clinical consequences: a 47-year-old man went through seven rounds of primaquine-based therapy for vivax malaria, relapsing every time, before clinicians discovered he carried a CYP2D6 mutation impairing activation of the drug.27Hindawi / PubMed Central. Relapsing Malaria: A Case Report of Primaquine Resistance In such cases, increasing the total primaquine dose may help overcome the metabolic bottleneck, though the evidence for this approach is still limited to small case series.
Primaquine in Children and Breastfeeding Women
Children carry a disproportionate burden of vivax malaria, yet dosing primaquine in young children is tricky. Pharmacokinetic data in children under five are scarce, and pediatric formulations that allow precise dosing are not widely available.28PubMed Central. Plasmodium vivax in Children: Hidden Burden and Conspicuous Challenges, a Narrative Review Tablets must often be split or crushed, introducing dosing errors. Combined with the difficulty of G6PD testing in young children, many clinicians simply avoid prescribing primaquine to this group, leaving the children at risk of repeated relapses.29PubMed Central. Anti-infectivity efficacy and pharmacokinetics of WHO recommended single low-dose primaquine in children with acute Plasmodium falciparum in Burkina Faso: study protocol – Section: BACKGROUND
For breastfeeding women, the WHO’s 2023 treatment guidelines restrict primaquine use when the nursing infant is under six months old, or when the infant’s G6PD status is unknown or deficient. The concern is that primaquine metabolites excreted in breast milk could cause hemolysis in a G6PD-deficient baby.30PubMed Central. Addressing health equity for breastfeeding women: primaquine for Plasmodium vivax radical cure However, a pharmacokinetic study measuring primaquine concentrations in breastfed infants found that nearly all samples were below the limit of detection, and no evidence of drug-related hemolysis was observed in the infants.31PubMed Central. Primaquine Pharmacokinetics in Lactating Women and Breastfed Infant Exposures The restriction, while cautious, means many breastfeeding women in endemic areas go without radical cure for months or years, accumulating relapses. Researchers have argued this represents a health-equity gap that deserves more attention and more pharmacokinetic data to inform policy.
Geographic Variation in G6PD Variants
Understanding the local G6PD variant landscape matters more than clinicians sometimes appreciate. In sub-Saharan Africa, the predominant A- variant causes moderate enzyme deficiency, and the hemolytic risk from standard primaquine doses, while real, tends to be less severe than with variants common elsewhere. In western Asia, the Mediterranean variant can leave people with almost no residual enzyme activity, making even a single dose of primaquine potentially dangerous. Across China and Southeast Asia, the picture is more fragmented, with a wide variety of mutations and no single dominant variant.16PubMed Central. Spatial distribution of G6PD deficiency variants across malaria-endemic regions A clinician deciding whether a weekly primaquine regimen is safe enough for a confirmed G6PD-deficient patient ideally wants to know which variant the patient carries, not just that they are deficient. In practice, variant-level genotyping is rarely available outside research settings, so decisions get made based on population-level knowledge and close clinical monitoring.