A “G6 drug” refers to a medication that can be dangerous for people with G6PD deficiency, a genetic condition affecting red blood cells. G6PD (glucose-6-phosphate dehydrogenase) is an enzyme that protects red blood cells from damage, and roughly 400 million people worldwide lack enough of it. When someone with this deficiency takes certain medications, their red blood cells can break apart rapidly, causing a potentially serious condition called hemolytic anemia.
The World Health Organization estimates that 7.5% of the global population carries the gene for G6PD deficiency, and about 2.9% are actively deficient. Because the gene sits on the X chromosome, it affects men more often than women. If you’ve heard the term “G6 drug,” it almost certainly came up in the context of medications these individuals need to avoid.
How G6PD Deficiency Makes Certain Drugs Dangerous
Red blood cells are unusually vulnerable cells. They have no nucleus and can’t repair themselves the way most other cells can. They rely entirely on G6PD to generate a protective molecule called NADPH, which in turn keeps an antioxidant system running inside the cell. That system neutralizes harmful byproducts (reactive oxygen species) before they can damage the cell membrane.
When someone with low G6PD activity takes a high-risk drug, their body converts that drug into metabolites that flood red blood cells with oxidative stress. Without enough of the protective antioxidant system running, the damage cascades quickly. Hemoglobin inside the cell clumps into formations called Heinz bodies, key structural proteins in the cell membrane become damaged and unstable, and the red blood cell essentially falls apart. The immune system then clears these damaged cells faster than the body can replace them, leading to anemia.
This process can begin within hours of taking the medication and progress rapidly. Symptoms include fatigue, dizziness, shortness of breath, jaundice (yellowing of the skin and eyes), and dark or bloody urine. In severe cases, the rapid destruction of red blood cells can become life-threatening.
High-Risk Medications to Know
Not all drugs pose equal risk. The medications most strongly linked to hemolytic crises in G6PD-deficient people fall into a few categories:
- Antimalarials: Primaquine (at standard doses) and tafenoquine are the most well-known triggers. These drugs are so consistently problematic that G6PD testing is now required before prescribing them in many countries.
- Dapsone: Used for certain skin conditions and infections, dapsone is considered high-risk. Its metabolite, dapsone hydroxylamine, directly attacks red blood cell membranes.
- Rasburicase and pegloticase: These are used to treat severe gout or high uric acid levels and carry clear warnings about G6PD deficiency.
Medium-risk medications include nitrofurantoin (a common urinary tract infection antibiotic) and phenazopyridine (a bladder pain reliever). These may or may not cause problems depending on the severity of someone’s deficiency and the dose used.
Drugs Often Mistaken as Dangerous
There’s significant confusion around which drugs are actually unsafe. Antibiotics are one class where this confusion runs deep. Over the years, many antibiotics have been added to and removed from avoidance lists, largely because the hemolysis people experienced was more likely caused by the infection itself rather than the antibiotic treating it. Infections are themselves a major trigger for hemolytic episodes in G6PD-deficient individuals, which muddies the picture.
Several common medications are considered reasonably safe at normal therapeutic doses for people with G6PD deficiency who don’t have the most severe form (chronic hemolytic anemia). These include acetaminophen, aspirin, chloroquine, quinine, trimethoprim, isoniazid, and vitamin K. “Reasonably safe” means they can be used at standard doses with monitoring, though higher-than-normal doses may still pose risk. For people with the most severe variant of the deficiency, even these drugs may not be safe.
Severity Varies by Genetic Variant
G6PD deficiency isn’t one-size-fits-all. The WHO classifies genetic variants into distinct groups based on how much enzyme activity remains and what symptoms result:
- Class A: Less than 20% of normal enzyme activity, associated with chronic hemolytic anemia that persists even without triggers.
- Class B: Less than 45% of normal activity, causing acute hemolytic episodes only when exposed to triggers like drugs, fava beans, or infections.
- Class C: Greater than 60% of normal activity, with no hemolysis. These individuals are essentially unaffected.
- Class U: Variants where clinical significance is still uncertain.
Most people with G6PD deficiency fall into Class B. They live completely normal lives until they encounter a trigger. Someone in this group might never know they have the condition until they take a specific medication and develop unexpected symptoms. Class A is rarer and more serious, with ongoing red blood cell destruction that can worsen dramatically with drug exposure.
Testing Before Prescribing
G6PD status can be checked with a simple blood test. Qualitative screening tests give a yes-or-no answer about whether someone is deficient, while quantitative tests measure the exact level of enzyme activity. Testing is particularly important before prescribing antimalarials like primaquine or tafenoquine, where the FDA requires confirmed G6PD status before dispensing.
One important caveat: testing right after a hemolytic episode can give a falsely normal result. That’s because the oldest, most deficient red blood cells have already been destroyed, leaving behind younger cells with higher enzyme levels. If testing is done during or shortly after a crisis, it may need to be repeated weeks later once the blood cell population has stabilized.
Triggers Beyond Medications
Drugs aren’t the only concern. Fava beans (broad beans) are a well-established dietary trigger, and the connection is so historic that G6PD deficiency is sometimes called “favism.” Infections, particularly bacterial ones, are actually the most common cause of hemolytic episodes worldwide, more common even than drug reactions. The oxidative stress generated by the body’s immune response during an infection can overwhelm G6PD-deficient red blood cells in the same way a high-risk drug would.
For people living with G6PD deficiency, knowing your status and sharing it with every healthcare provider and pharmacist is the most effective way to avoid a hemolytic crisis. Pharmacists are specifically trained to cross-check new prescriptions against a patient’s G6PD status and flag medications with oxidant or hemolytic potential before they’re dispensed.