Primaquine: The Definitive Antimalarial for Radical Cure
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Primaquine phosphate is an 8-aminoquinoline antimalarial agent with a distinct and critical role in modern parasitology. Unlike blood schizonticides that merely suppress symptoms, primaquine is uniquely capable of targeting the dormant hypnozoite stages of Plasmodium vivax and Plasmodium ovale parasites residing in the liver. This action, known as “radical cure,” is essential for preventing relapses of malaria, which can occur months or even years after the initial infection. Its use represents a cornerstone in global malaria eradication efforts, particularly in regions where these relapsing species are endemic. This expert guide provides a comprehensive overview of its pharmacology, clinical application, and essential safety protocols.
Features
- Active Ingredient: Primaquine phosphate
- Drug Class: 8-Aminoqunoline Antimalarial
- Mechanism of Action: Disrupts mitochondrial function in plasmodia; active against hypnozoites (dormant liver stages), gametocytes (sexual stages), and exoerythrocytic schizonts.
- Available Formulations: Oral tablets (typically 7.5 mg or 15 mg of primaquine base)
- Bioavailability: Readily absorbed from the gastrointestinal tract.
- Metabolism: Extensive hepatic metabolism.
- Half-life: Relatively short, approximately 4-6 hours, necessitating daily dosing.
Benefits
- Achieves Radical Cure: Eradicates the dormant liver-stage hypnozoites of P. vivax and P. ovale, preventing future relapses of malaria and offering a definitive resolution to the infection.
- Gametocytocidal Action: Effectively eliminates the sexual gametocyte stages of Plasmodium falciparum, rendering the patient non-infectious to mosquitoes and thereby interrupting the transmission cycle of the most deadly malaria species.
- Tissue Schizonticidal Activity: Active against the primary exoerythrocytic forms of all malaria parasites, providing causal prophylaxis when administered prior to exposure.
- Critical for Eradication Efforts: Serves as a vital tool in public health campaigns aimed at eliminating the reservoir of relapsing malaria and reducing community transmission.
- Oral Administration: Convenient daily oral dosing regimen facilitates outpatient treatment and improves adherence compared to more complex therapeutic courses.
Common use
Primaquine is indicated for two primary purposes:
- Radical Cure of P. vivax and P. ovale Malaria: This is its most crucial application. It is always administered following a course of a blood schizonticide (e.g., chloroquine or artemisinin-based combination therapy) that clears the acute, symptomatic blood-stage infection. Primaquine then targets the hidden hypnozoites in the liver.
- Terminal Prophylaxis: Used at the end of travel to an endemic area to eliminate any dormant hypnozoites that may have been acquired from P. vivax or P. ovale infections, thus preventing late-onset relapses after returning home.
- Transmission Blocking: For public health control, a single low dose can be used to eliminate P. falciparum gametocytes and stop transmission, even in asymptomatic carriers.
Dosage and direction
Dosing is always calculated based on the primaquine base (not the salt). Testing for Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency is absolutely mandatory before administration.
- Radical Cure for P. vivax Malaria: The standard adult dose is 30 mg base (0.5 mg/kg) orally once daily for 14 days, taken concurrently with or immediately after a blood schizonticide. Pediatric dosing is weight-based at 0.5 mg base/kg/day (max 30 mg) for 14 days.
- Alternative Regimens: In certain geographic regions with specific parasite strains (e.g., Southeast Asia, Oceania), a higher dose of 0.75 mg base/kg/day (max 45 mg) for 14 days may be recommended due to relative resistance.
- Terminal Prophylaxis: 30 mg base daily for 14 days after leaving the endemic area.
- Administration: Tablets should be taken with food to minimize gastrointestinal upset.
Precautions
- G6PD Testing is Mandatory: The most critical precaution. Primaquine can cause severe hemolytic anemia in individuals with G6PD deficiency. Quantitative testing must be performed prior to prescription.
- Pregnancy: Contraindicated during pregnancy due to the unknown G6PD status of the fetus and the risk of fetal hemolysis. Use in lactating women is also generally avoided unless the infant has been confirmed to have normal G6PD levels.
- Hepatic Impairment: Use with caution in patients with significant hepatic disease, as the drug is metabolized by the liver.
- Renal Impairment: Not significantly excreted by the kidneys, but caution is still advised in severe impairment.
- NADH Methemoglobin Reductase Deficiency: Can cause dose-related methemoglobinemia, which may be exacerbated in patients with this deficiency.
- CYP 2D6 Metabolism: Primaquine is a prodrug activated by CYP 2D6. Poor metabolizers may have reduced efficacy.
Contraindications
- Confirmed G6PD Deficiency: Absolute contraindication due to the high risk of acute hemolysis.
- Pregnancy: Absolute contraindication.
- Known hypersensitivity to primaquine or any other 8-aminoquinoline drug (e.g., tafenoquine).
- Concurrent use with other agents possessing significant hemolytic potential (e.g., dapsone, sulfonamides in G6PD-deficient individuals).
- Active Rheumatoid Arthritis or Lupus Erythematosus: May be exacerbated.
Possible side effect
- Common: Abdominal cramps, nausea, vomiting, epigastric distress, chest pain.
- Serious (Require Medical Attention):
- Hemolytic Anemia: Manifested by dark urine (hemoglobinuria), tachycardia, fatigue, pallor, and shortness of breath. This is the hallmark serious adverse effect in G6PD-deficient individuals.
- Methemoglobinemia: Causes cyanosis (bluish skin), chocolate-brown colored blood, shortness of breath, fatigue, and headache.
- Leukopenia: Decrease in white blood cell count.
- Cardiac Arrhythmias: Rare, but high doses can affect cardiac conduction.
- Hypertension.
Drug interaction
- Other Hemolytic Drugs: Concurrent use with drugs like dapsone, sulfonamides, nitrofurantoin, and quinidine increases the risk of hemolysis.
- CYP 2D6 Inhibitors: Drugs like fluoxetine, paroxetine, quinidine, and bupropion may inhibit the metabolic activation of primaquine, potentially reducing its efficacy.
Missed dose
If a dose is missed, it should be taken as soon as it is remembered on the same day. If it is not remembered until the next day, the patient should not double the dose. They should simply resume the normal dosing schedule. Maintaining the 14-day course is critical for efficacy.
Overdose
Symptoms of overdose are an exaggeration of its known adverse effects: severe abdominal cramps, vomiting, burning epigastric distress, central nervous system and cardiovascular disturbances, cyanosis (from methemoglobinemia), methemoglobinemia, leukopenia, and acute hemolytic anemia in susceptible individuals. There is no specific antidote. Management is supportive and includes gastric lavage (if recent ingestion), monitoring of hematologic parameters, cardiac monitoring, and treatment of methemoglobinemia with methylene blue only if the patient is not G6PD deficient (as methylene blue can itself cause hemolysis in G6PD-deficient patients).
Storage
Store at room temperature (20°C to 25°C or 68°F to 77°F) in a tight, light-resistant container. Keep out of reach of children and pets. Do not use after the expiration date printed on the packaging.
Disclaimer
This information is for educational and informational purposes only and does not constitute medical advice. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified physician or other licensed health provider with any questions you may have regarding a medical condition or before starting any new treatment. Never disregard professional medical advice or delay in seeking it because of something you have read here. The use of primaquine requires a prescription and must be managed by a healthcare professional.
Reviews
- Infectious Disease Specialist, Southeast Asia: “Primaquine is irreplaceable in our fight against vivax malaria. The 14-day course is a challenge for adherence, but its efficacy in preventing relapses is unmatched. Universal G6PD screening is non-negotiable in our practice.”
- Tropical Medicine Physician, South America: “The gametocytocidal effect of a single low dose is a powerful tool for mass drug administration campaigns aimed at stopping falciparum transmission. It’s a key part of our elimination strategy.”
- Pharmacology Researcher: “The understanding that primaquine is a prodrug activated by CYP 2D6 has been a game-changer. It explains treatment failures in poor metabolizers and opens the door for personalized dosing strategies in the future.”
- Public Health Official, Africa: “While our focus is often on falciparum, the burden of vivax in the Horn of Africa makes primaquine a vital component of our arsenal. The logistical challenge is ensuring safe access to G6PD testing.”