How Toxic Is Leflunomide? Liver, Blood, and Skin Risks

Leflunomide is a genuinely useful drug for rheumatoid arthritis, but it carries a toxicity profile that demands respect. It can damage the liver, suppress blood cell production, trigger severe skin reactions, and harm a developing fetus. Most people tolerate it well enough at standard doses, yet real-world data paint a sobering picture: roughly four out of ten patients stop taking leflunomide within the first year because of side effects, a rate significantly higher than for methotrexate, the most commonly used alternative. Understanding which risks are common nuisances and which are rare emergencies can help you have a more informed conversation with your rheumatologist about whether the drug is right for you.

How Leflunomide Works and Why That Matters for Toxicity

Leflunomide blocks an enzyme involved in building pyrimidine nucleotides, which are essential raw materials for DNA and RNA synthesis. By starving rapidly dividing immune cells of these building blocks, the drug slows down the overactive immune response that drives rheumatoid arthritis.¹ The problem is that this mechanism is not perfectly selective. Other cell types that divide quickly, including bone marrow cells that produce blood components, can also be caught in the crossfire. That same lack of selectivity helps explain the drug’s range of side effects across multiple organ systems.

Another complication is how long leflunomide stays in your body. The drug converts into an active metabolite called teriflunomide, which recirculates through the liver and binds tightly to proteins in the blood. This means the active compound can linger for weeks or even months after you stop taking the pills. Genetic differences in liver enzymes like CYP1A2 and CYP2C19 contribute to wide variation in blood levels between patients, which partly explains why one person sails through treatment while another develops toxicity.² That slow clearance is why a special washout procedure exists, using cholestyramine or activated charcoal to flush the drug from your system when serious side effects appear.

Liver Damage

The liver is where leflunomide’s toxicity story gets the most attention, and for good reason. The drug is metabolized in the liver, and its active metabolite recirculates through liver tissue repeatedly. Animal studies show that leflunomide significantly raises liver enzymes (AST and ALT) and causes visible inflammation and cell death in liver tissue.³ Lab work has identified several pathways involved, including increased oxidative stress and depletion of the liver’s natural antioxidant defenses.⁴

In clinical practice, elevated liver enzymes are one of the most common reasons patients stop leflunomide. A large cohort study found that about 9% of patients discontinued leflunomide with abnormal blood tests within the first year, and roughly 20% did so within five years. Having abnormal liver enzymes or low blood counts during the first six months of treatment was a strong predictor of eventually needing to stop the drug altogether.⁵

Severe liver injury is less common but can be life-threatening. A comparison of liver injury cases from registries in India and the United States found that the predominant pattern was hepatocellular damage, meaning the injury targeted the liver cells themselves rather than the bile ducts. Women were disproportionately affected by this pattern, and hepatocellular injury was associated with younger age and higher mortality compared to other injury patterns.⁶ That mortality finding is worth pausing on: when leflunomide does cause serious hepatocellular damage, the consequences can be fatal.

Pre-existing liver conditions raise the stakes. A case report documented liver toxicity when leflunomide was combined with methotrexate in a patient who also had non-alcoholic fatty liver disease, suggesting that underlying liver fat may make the liver more vulnerable to drug-induced injury.⁷ This is why doctors generally insist on liver function tests before starting leflunomide and at regular intervals throughout treatment.

Blood Cell Suppression

Because leflunomide inhibits the machinery that dividing cells use to make DNA, it can suppress the bone marrow’s ability to produce blood cells. The most dangerous version of this is pancytopenia, where red blood cells, white blood cells, and platelets all drop simultaneously. This is rare but can become a medical emergency very quickly, since a crash in white blood cells leaves you unable to fight infections while a drop in platelets increases bleeding risk.

Australian post-marketing surveillance data estimated that pancytopenia occurred in roughly one out of every 3,700 to 4,600 patients taking leflunomide alone. When methotrexate was added, the risk climbed sharply, to about one in 575 to 822 patients.⁸ That four-to-sixfold increase in risk with combination therapy is clinically significant, and it is one reason doctors monitor blood counts more closely when both drugs are prescribed together.⁹

Case reports illustrate just how serious this complication can become. One case described a 61-year-old man who developed profound pancytopenia and agranulocytosis after just two months of leflunomide monotherapy, presenting in septic shock and requiring vasopressors and mechanical ventilation.¹⁰ The timing is unpredictable. Blood counts can drop within weeks or months after starting the drug, which is why ongoing monitoring matters even after you have been stable on it for a while.

Skin Reactions

Mild skin side effects like rash and hair thinning are fairly common with leflunomide and are usually manageable. The serious concern is at the other end of the severity spectrum: Stevens-Johnson syndrome and toxic epidermal necrolysis. These are immune-mediated reactions where the skin essentially blisters and peels away from the body, often involving mucous membranes as well. They are medical emergencies with significant mortality rates.

Both Stevens-Johnson syndrome and toxic epidermal necrolysis have been reported with leflunomide, though they remain rare.¹¹ When they occur, stopping the drug immediately is only the first step. Because leflunomide’s active metabolite lingers in the body for so long, a cholestyramine washout must be started right away to accelerate drug elimination.¹² Other serious cutaneous reactions reported include vasculitis and lichenoid drug rashes.¹³

There is also a less dramatic but still troubling skin reaction: drug-induced subacute cutaneous lupus erythematosus. This presents as a widespread rash with features resembling lupus, sometimes accompanied by erythema multiforme-like lesions. Cases have been reported in patients with rheumatoid arthritis and ankylosing spondylitis, and withdrawal of leflunomide combined with a short course of steroids typically resolves the rash.¹⁴¹⁵ The takeaway is that any new or worsening rash during leflunomide treatment deserves prompt medical attention, since distinguishing a benign drug rash from the early stages of a severe reaction can be difficult without a clinical assessment.

Peripheral Neuropathy

Tingling, numbness, or pain in the hands and feet is a side effect that often catches patients off guard because it is not as prominently discussed as the liver and blood risks. A New Zealand cohort study found that about 5% of patients on leflunomide developed peripheral neuropathy.¹⁶ Symptoms typically started around six months into treatment, though the range spanned from a few days to several years.¹⁷

The neuropathy usually follows a pattern of sensory loss in the feet and hands, sometimes extending up the calves and forearms in a “glove-and-stocking” distribution. Beyond numbness, patients have reported pain, poor sleep, balance problems, and compromised skin integrity from the nerve damage.¹⁶ The encouraging finding is that the vast majority of patients improved or stabilized after stopping the drug. A review of 80 reported cases found that patients who discontinued leflunomide within 30 days of symptom onset were significantly more likely to recover than those who continued taking it longer.¹⁷ A cholestyramine washout can speed this process along when neuropathy progresses quickly.¹⁸

Lung and Blood Pressure Effects

Interstitial lung disease is a rare but potentially fatal complication. A systematic review found that most cases appeared within the first three months of treatment, with patients presenting with acute respiratory symptoms lasting a week or less. Imaging typically showed bilateral ground-glass opacities, and the most common tissue finding was diffuse alveolar damage. Patients with pre-existing lung disease were at particular risk, and the review concluded that leflunomide should be avoided in this population entirely.¹⁹ Interestingly, the condition has been reported mainly in Asian populations, and at least one case occurred even in a patient with no underlying lung disease, following an accidental overdose while also taking methotrexate.²⁰

A more common cardiovascular side effect is raised blood pressure. Leflunomide-related hypertension has been noted in up to 10% of patients, either worsening pre-existing hypertension or causing new-onset high blood pressure within the first few months of therapy.²¹ A comparative study found that patients starting leflunomide had about a 50% greater risk of developing new hypertension compared to those starting methotrexate.²² The exact mechanism remains unclear, though increased sympathetic nervous system activity has been proposed. Most clinicians consider this manageable with antihypertensive medication or dose adjustments, but it does mean your blood pressure should be checked regularly during treatment.

How Leflunomide Compares to Methotrexate

Since methotrexate is the most common first-line drug for rheumatoid arthritis, patients and doctors frequently weigh the two drugs against each other. A two-year head-to-head trial found that the most common side effects overlapped: diarrhea, nausea, hair loss, rash, headache, and elevated liver enzymes occurred in both groups. However, methotrexate actually led to more withdrawals due to elevated liver enzymes than leflunomide did. The methotrexate group also had two drug-related deaths from pulmonary causes, while the leflunomide group had none.²³

That might sound like a win for leflunomide, but the broader picture is more complicated. Norwegian registry data showed that the one-year discontinuation rate due to adverse events was about 43% for leflunomide compared to roughly 14% for methotrexate.²⁴ A French observational study found a similar pattern, with about 57% of leflunomide patients discontinuing within a year, mainly due to side effects.²⁵ A Dutch prospective study reported that 56% of patients withdrew during follow-up, with adverse drug reactions accounting for the majority of those withdrawals.²⁶

The gap is striking. Leflunomide is not more dangerous than methotrexate in any single category of serious adverse events, but it produces more everyday side effects that patients find intolerable enough to quit. Diarrhea in particular is notably more common with leflunomide and drives a significant share of those early discontinuations. For patients who do tolerate the drug past the first several months, long-term safety appears more manageable.

Pregnancy and Reproductive Safety

Leflunomide is classified as a known teratogen. In animal studies, the drug caused severe birth defects including skull malformations, exposed brain tissue, and abnormalities of the vertebrae and limbs.²⁷ Women taking leflunomide are advised to use reliable contraception and to avoid becoming pregnant while on the drug.²⁸

If pregnancy does occur, a cholestyramine washout procedure is recommended to reduce fetal exposure as quickly as possible. The drug’s long half-life makes this especially urgent: without the washout, detectable levels of the active metabolite can persist in the bloodstream for months. Human data on actual birth outcomes remain limited, and the risk of major malformations in human pregnancies exposed to leflunomide has been studied but is not fully settled.²⁹ The precautionary stance is firm: stop the drug and wash it out if pregnancy is planned or discovered.

The Washout Procedure and Why It Exists

Leflunomide is unusual among rheumatoid arthritis drugs in having a dedicated washout protocol. The standard approach uses cholestyramine, taken three times daily for 11 days, or alternatively activated charcoal. Both work by binding the drug’s active metabolite in the gut and preventing it from being reabsorbed through the liver’s recirculation pathway.¹² Without the washout, it can take up to two years for blood levels to fall below detectable thresholds on their own.

The washout is not reserved only for severe reactions. It is recommended before pregnancy, when switching to certain other medications that could interact with lingering teriflunomide, and whenever a serious adverse event occurs. The speed at which it clears the drug can be clinically meaningful: in cases of rapidly progressing peripheral neuropathy, for example, cholestyramine washout has been associated with faster resolution of nerve symptoms than stopping the drug alone.¹⁸

Monitoring During Treatment

Current guidelines recommend blood tests every three months during long-term leflunomide treatment, with more frequent checks when a patient is also on other medications or has conditions that increase risk.⁵ These tests typically include a complete blood count to catch early signs of bone marrow suppression and liver function tests to monitor for hepatotoxicity.

The first six months appear to be a critical window. Having either low blood counts or elevated liver enzymes during this period was a strong predictor of eventually needing to discontinue the drug, with the risk of later drug cessation roughly tripled in patients who showed early abnormal results.⁵ Blood pressure should also be checked regularly, given the 10% incidence of treatment-related hypertension. If you are on leflunomide and your doctor is not ordering regular blood work, that is worth bringing up proactively.

Genetic Variation and Individual Susceptibility

One of the frustrating aspects of leflunomide toxicity is how unpredictable it can be. Two patients on the same dose may have dramatically different blood levels of the active metabolite, owing to genetic variation in the liver enzymes that process the drug. Researchers have investigated whether specific genetic variants might predict who will develop side effects. One study looked at polymorphisms in CYP2C19 and CYP2C9 and found no significant association with leflunomide toxicity.³⁰ Other enzymes like CYP1A2 and a transporter called ABCG2 have also been studied as potential contributors to the wide variation in drug levels between patients.²

At this point, there is no routine genetic test that can tell you before starting leflunomide whether you are likely to develop a serious reaction. The science here is still early, and in the meantime, regular blood monitoring remains the most practical way to catch problems before they become dangerous. Combination therapy with methotrexate, pre-existing liver disease, and older age remain the most reliable clinical risk factors that doctors weigh when deciding whether and how to prescribe leflunomide.

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