What Is Diclofenac Sodium and How Does It Work?

Diclofenac sodium is one of the most widely used anti-inflammatory and pain-relieving drugs in the world, belonging to the family of nonsteroidal anti-inflammatory drugs (NSAIDs). Originally developed in the 1970s to treat rheumatic conditions, it has since become a go-to medication for everything from osteoarthritis and post-surgical pain to sports injuries. But diclofenac carries a more complicated profile than many people realize, with cardiovascular risks that prompted European regulators to add new contraindications in 2013 and an environmental legacy that nearly drove several vulture species to extinction.

How Diclofenac Works

Like other NSAIDs, diclofenac reduces pain and inflammation by blocking enzymes called cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2). These enzymes help produce prostaglandins, which are chemicals that trigger inflammation, pain, and fever. What sets diclofenac apart from drugs like ibuprofen or naproxen is its selectivity profile. While ibuprofen and naproxen are essentially nonselective, blocking both COX enzymes roughly equally, diclofenac leans toward COX-2 selectivity.¹ That COX-2 preference is relevant both for its effectiveness and for the cardiovascular concerns discussed below.

Beyond the standard COX-blocking pathway, researchers have found that diclofenac may also affect various ion channels in the body, which could contribute to its pain-relieving effects through routes that are not fully understood.² This additional activity may help explain why some patients find diclofenac more effective than other NSAIDs at similar doses, though the clinical significance of these non-COX effects is still being studied.

Available Forms and Why They Matter

Diclofenac comes in oral tablets, topical gels, solutions, patches, suppositories, and even ophthalmic eye drops. The choice of formulation is not just about convenience; it fundamentally changes what happens inside your body. In healthy volunteers, the systemic blood levels from a 1% diclofenac gel were 5 to 17 times lower than from an oral dose.³ More recent research comparing topical diclofenac diethylamine gels with oral tablets found systemic concentrations up to 75- to 150-fold lower with topical application.⁴

This dramatic difference in blood levels has practical consequences. The oral form inhibited platelet aggregation and strongly suppressed both COX enzymes throughout the body, while topical diclofenac did not inhibit platelet aggregation and had much less systemic COX suppression.⁵ Available data also suggest that topical diclofenac can penetrate into deeper tissues beneath the skin, achieving a lower ratio of plasma concentration to tissue concentration compared with the oral form.⁶ In plain terms, the gel gets more drug where you need it and less where you don’t.

Effectiveness for Osteoarthritis Pain

The strongest evidence for diclofenac’s clinical value comes from osteoarthritis of the knee and hand. A network meta-analysis of randomized trials found that topical diclofenac, whether as gel, solution, or patch, significantly outperformed placebo for both pain relief and physical function. The patches showed the strongest short-term effect among all topical NSAIDs studied.⁷ A separate meta-analysis confirmed that gel and solution formulations maintained their benefits over mid-term and long-term treatment periods of up to 12 weeks, while skin reactions and systemic side effects remained low across formulations.⁸

One question that comes up often is whether topical diclofenac is a poor substitute for oral NSAIDs or genuinely competitive. Three randomized trials directly compared the two, and all three found topical diclofenac provided at least equivalent relief of osteoarthritis pain and other symptoms.⁹ In systematic reviews, roughly 43% to 59% of patients using topical diclofenac achieved clinically meaningful pain relief (at least a 50% reduction in pain), compared with 23% to 48% for placebo.¹⁰

Pooled data from three large trials also showed that topical diclofenac sodium gel was effective regardless of age, with patients 65 and older achieving significant improvements in pain scores and physical function compared with vehicle alone over 12 weeks.¹¹ Given that older adults are at higher risk for the gastrointestinal and cardiovascular side effects of oral NSAIDs, this age-independent efficacy makes the topical form especially appealing for that population.

The Cardiovascular Risk

This is where diclofenac’s story gets complicated. A large Danish cohort study found that people who started diclofenac had about a 50% higher rate of major adverse cardiovascular events compared with people who did not start any NSAID, roughly a 20% higher rate compared with those who started ibuprofen or paracetamol (acetaminophen), and about a 30% higher rate compared with naproxen starters. These increases held even at low doses.¹² Looking at specific outcomes, the risks were elevated for heart attack, heart failure, stroke, irregular heart rhythm, and cardiac death.

A follow-up study using emulated trials across the Danish healthcare system compared diclofenac with other COX-2-selective NSAIDs. Diclofenac users had about a 20% increase in major cardiovascular events compared with users of older COX-2-selective drugs like meloxicam or etodolac. The effect was strongest among women, people with high baseline cardiovascular risk, and those taking higher doses. When compared head-to-head with the newer COX-2 inhibitors celecoxib and etoricoxib, however, diclofenac showed similar cardiovascular risk rates.¹³

Some nuance exists in these findings. An updated review noted that while diclofenac appeared to carry a relatively higher heart attack risk among NSAIDs, one large study found that the risk actually decreased with increasing baseline cardiovascular risk, to the point that in patients already at high cardiovascular risk, diclofenac was no more dangerous than paracetamol or other NSAIDs.¹⁴ This somewhat counterintuitive result does not mean high-risk patients should be careless with diclofenac; it means the relative risk picture depends on what you compare it to and who you study. In 2013, the European Medicines Agency concluded that diclofenac posed an elevated cardiovascular risk and issued new contraindications across the EU.¹⁵

Gastrointestinal, Liver, and Kidney Risks

The GI effects are perhaps the most familiar NSAID side effect. Diclofenac, like its relatives, can damage the stomach lining. Endoscopic studies in healthy volunteers have shown that the soluble form of diclofenac can directly cause gastroduodenal injury, including erosions and ulcers, and delay the healing of existing lesions.¹⁶ This is a local topical effect on the gut mucosa on top of the systemic prostaglandin reduction, meaning the GI risk from oral diclofenac comes from two directions at once.

Liver injury from diclofenac is uncommon but well-documented. The drug is metabolized in the liver to reactive intermediates that can bind to proteins and trigger immune-mediated damage. Research using both rat and human liver cells has identified reactive acylating metabolites of diclofenac, beyond just its glucuronide conjugate, that may play a role in this idiosyncratic liver toxicity.¹⁷ The word “idiosyncratic” here matters: it means that liver damage is unpredictable and not simply dose-related, which is why liver function monitoring is sometimes recommended during prolonged oral diclofenac use.

Kidney damage is another concern, particularly for people who are dehydrated, elderly, or have high blood pressure. NSAIDs as a class account for a substantial share of drug-induced kidney failure.¹⁸ Animal studies have shown that diclofenac causes more pronounced kidney damage and electrolyte disturbances in hypertensive animals than in normotensive ones, reinforcing the clinical observation that people with high blood pressure are at greater risk.¹⁹

Interactions with Blood Thinners

If you take warfarin or another anticoagulant, diclofenac deserves extra caution. All NSAIDs can prolong bleeding time by impairing platelet function, and they can cause gastrointestinal bleeding that becomes more severe when a blood thinner is on board. While the importance of the protein-binding displacement interaction between warfarin and NSAIDs has been overstated in the past, it can become clinically relevant at high NSAID concentrations or in people who metabolize warfarin slowly, such as those with significant heart failure or impaired liver function.²⁰ The combination is not absolutely prohibited, but it requires close monitoring and ideally the shortest possible duration.

Eye Drops and a Rare but Serious Complication

Diclofenac sodium 0.1% eye drops are prescribed after cataract surgery and other eye procedures to control inflammation and pain. They work well for most people, but a rare and serious complication called corneal melting has been reported. A case series described 11 patients who developed severe corneal toxicity following the use of 0.5% diclofenac ophthalmic solution, with the time from first use to corneal melting ranging from six days to 17 months.²¹ Coexisting conditions and treatments complicated the picture, making it hard to attribute the damage solely to diclofenac. More recently, a case report described corneal melting in a patient with rheumatoid arthritis after eye surgery, reinforcing that people with autoimmune diseases may be at higher risk for this complication.²²

The Vulture Crisis

Perhaps the most dramatic chapter in diclofenac’s history has nothing to do with human medicine. In the 1990s, populations of three species of Gyps vultures in South Asia began collapsing at catastrophic speed. Researchers eventually traced the cause to diclofenac used in veterinary medicine to treat cattle. When vultures fed on the carcasses of livestock that had been treated with the drug shortly before death, they suffered acute kidney failure and died. The mechanism involved increased production of reactive oxygen species in kidney cells and interference with uric acid transport, a combination that proved uniquely lethal to vultures.²³

The scale of the die-off was staggering. Populations of the oriental white-backed vulture, long-billed vulture, and slender-billed vulture declined by over 95% across the Indian subcontinent. India, Pakistan, and Nepal banned veterinary diclofenac in 2006. Researchers identified meloxicam, another NSAID, as a safe alternative after testing it in dozens of captive Gyps vultures with no ill effects and no elevation in uric acid levels.²⁴ Safety testing on additional scavenging bird species in India, including Egyptian vultures, cattle egrets, and crows, confirmed that meloxicam was well tolerated.²⁵

The ban worked, though slowly. Monitoring of long-billed vulture populations in Pakistan showed that nest occupancy and nest productivity, which had declined by 73% and 95% respectively in the three years before the ban, rebounded by 52% and 95% within one to two years after the ban. The vast majority of observed vulture deaths had occurred before the ban took effect.²⁶ Modeling in India suggested that while the rate of decline had slowed to about 40% of what it had been before the ban, the oriental white-backed vulture was still declining at roughly 18% per year, partly because illegal diclofenac use persisted.²⁷

Diclofenac in the Environment

The vulture crisis drew attention to diclofenac as an environmental contaminant, and the drug turns out to be a problem in waterways as well. Because people excrete the drug and its metabolites after use, diclofenac enters sewage systems continuously. Conventional wastewater treatment plants remove it poorly. A study of sewage treatment plants in Ghana found an average removal efficiency of only about 74% for diclofenac, compared with 99% for ibuprofen and 98% for paracetamol.²⁸ What survives treatment ends up in rivers and lakes.

Even at low concentrations detected in European surface waters, diclofenac affects aquatic life. In rainbow trout, exposures at concentrations as low as 5 micrograms per liter caused kidney damage and destruction of gill tissue, with the drug accumulating at the highest levels in the liver.²⁹ Gene expression studies found that diclofenac altered hundreds of genes involved in inflammation and immune response in fish at blood plasma concentrations corresponding to about 1.5% of what humans achieve at therapeutic doses.³⁰ Brown trout exposed to the drug showed increased mortality at rising concentrations, with observable effects beginning at 10 micrograms per liter.³¹

The food chain question naturally follows. Research on chicory grown in diclofenac-contaminated soil found that the drug was absorbed through the roots and translocated to the aerial parts of the plant, though it did not bioaccumulate in the leaves. Based on estimated daily intake of chicory, irrigation with water containing diclofenac at up to 1 milligram per liter did not appear to pose a human health risk through consumption.³² That finding is somewhat reassuring for the agricultural angle, but the aquatic ecosystem effects remain a genuine concern, especially in rivers near major population centers where pharmaceutical residues concentrate.

Advanced wastewater treatment technologies like ozonation, activated carbon, and photocatalysis can achieve much higher removal rates, with some approaches reaching near-complete elimination in laboratory settings. However, scaling these up to real-world sewage systems is expensive and complex, and intermediate breakdown products can sometimes be formed that are not well characterized themselves.³³ For now, the EU included diclofenac on its first Watch List of priority substances for environmental monitoring in surface waters, signaling that regulators take the contamination seriously even if binding limits have not yet been imposed.

New Delivery Systems on the Horizon

Given diclofenac’s effectiveness alongside its systemic risks, researchers are trying to get the drug to where it is needed while minimizing its presence everywhere else. One approach uses microneedle patches loaded with diclofenac nanoparticles. In lab testing, these patches delivered the drug through the skin more effectively than conventional gel formulations, potentially enabling sustained local pain relief without the systemic drawbacks of oral dosing.³⁴ Another strategy developed a novel nanosuspension that disperses diclofenac sodium into an oil base, increasing skin permeation by nearly fourfold in laboratory models.³⁵ These technologies are early-stage and far from pharmacy shelves, but they reflect the broader drug-delivery philosophy: diclofenac itself is not the problem so much as where it ends up in the body.

Genetic Variation and Drug Metabolism

Diclofenac is primarily broken down in the liver by the enzyme CYP2C9. Several genetic variants of this enzyme exist that, in theory, could slow down the drug’s metabolism and lead to higher blood levels in certain people. Laboratory studies have predicted that carrying specific variant forms of CYP2C9 would reduce the rate at which diclofenac is cleared from the body.³⁶ In practice, though, the real-world picture is muddier. A study examining diclofenac metabolism in people with different CYP2C9 genotypes found wide variability within each genotype group but no significant differences between them, either in living subjects or in liver tissue samples.³⁷

What about the genetic link to liver injury? Researchers genotyped 24 patients who had developed diclofenac-induced hepatitis and compared them to 100 healthy controls, looking specifically at the CYP2C9 variants most commonly implicated. They found no evidence that these common variants made people more susceptible to diclofenac liver toxicity.³⁸ Full gene sequencing of the most severely affected patients also turned up no new mutations. So while CYP2C9 genetics remain relevant for many other drugs, they do not appear to explain who gets liver problems from diclofenac, which is why the term “idiosyncratic” persists in the literature. The search for a reliable biomarker to predict diclofenac-related liver damage continues, but for now, no genetic test can tell you whether you are at risk.