Diclofenac Sodium
Is It Right for You?
A complete guide to Diclofenac Sodium — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Diclofenac Sodium?
Diclofenac sodium is an orally administered non-steroidal anti-inflammatory drug (NSAID) with potent analgesic, anti-inflammatory, and antipyretic properties. It is one of the most widely used NSAIDs globally and is available in multiple formulations including standard immediate-release tablets (25 mg, 50 mg), modified-release capsules (75 mg, 100 mg), suppositories, topical gel (Voltarol Emulgel — available OTC for musculoskeletal pain), and injectable formulations. The brand names Voltarol and Dicloflex are well recognised in the UK.
Diclofenac belongs to the arylacetic acid subclass of NSAIDs and selectively inhibits cyclooxygenase (COX) enzymes — particularly COX-2 (the inducible isoform responsible for prostaglandin production at sites of inflammation and injury) more than COX-1 (the constitutive isoform expressed in gastric mucosa and platelets). This relative COX-2 selectivity is intermediate between non-selective NSAIDs (ibuprofen, naproxen) and highly selective COX-2 inhibitors (celecoxib, etoricoxib), and contributes to a somewhat lower risk of GI adverse effects compared with non-selective NSAIDs — though GI protection with a PPI is still recommended in high-risk patients.
What Conditions Does Diclofenac Treat?
Diclofenac is prescribed for:
Diclofenac intramuscularly (or via suppository) is a first-line treatment for the pain of renal colic (ureteric colic from kidney stones). It reduces prostaglandin-mediated ureteric spasm and associated loin-to-groin pain. It has been shown to be as effective as opioid analgesics for renal colic in controlled trials and avoids opioid-associated side effects.
Diclofenac is used as part of multimodal analgesia following surgical procedures, reducing opioid requirements and improving pain control.
Diclofenac potassium (faster-absorbing salt, available in some formulations) is used for acute migraine attacks. It reduces prostaglandin-mediated neurogenic inflammation in the trigeminovascular pathway.
While Diclofenac is effective for managing pain and inflammation, it should be used cautiously in patients with a history of gastrointestinal issues or cardiovascular disease.
How Does Diclofenac Work?
Arachidonic acid, released from membrane phospholipids by phospholipase A2 in response to tissue injury or inflammation, is converted by COX enzymes to prostaglandins (PGs), thromboxane A2 (TXA2), and prostacyclin (PGI2). Prostaglandins are key mediators of the cardinal signs of inflammation: they sensitise peripheral nociceptors (causing pain hypersensitivity), cause vasodilation (erythema), increase vascular permeability (oedema), and act centrally to raise body temperature (fever). Thromboxane A2 promotes platelet aggregation. Prostacyclin protects the gastric mucosa.
Diclofenac inhibits both COX-1 and COX-2 but with approximately 10-fold greater potency against COX-2. Inhibition of COX-2 reduces prostaglandin synthesis at inflammatory sites, producing the therapeutic anti-inflammatory and analgesic effects. Inhibition of COX-1 reduces protective prostaglandin production in the gastric mucosa (increasing ulcer risk) and reduces TXA2 in platelets (impairing platelet aggregation and prolonging bleeding time).
Diclofenac additionally inhibits the lipoxygenase pathway to some degree (reducing leukotriene synthesis), has membrane-stabilising properties, and modulates arachidonic acid metabolism via inhibition of phospholipase A2 at higher concentrations — contributing to additional anti-inflammatory effects beyond pure COX inhibition.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Always follow your clinician's instructions. Diclofenac should be taken with food to minimize gastrointestinal irritation. The modified-release formulations should be swallowed whole and not crushed or chewed.
Side Effects of Diclofenac
Diclofenac is generally well tolerated, but some side effects may occur.
- Gastrointestinal effects: Nausea, dyspepsia, abdominal pain, diarrhoea, and constipation are the most frequently reported adverse effects. Risk of GI ulceration, erosion, and bleeding is significant, particularly with higher doses, prolonged use, and in high-risk patients. GI side effects are reduced by taking with food and co-prescribing a PPI.
- Headache, dizziness
- Elevated liver enzymes: Usually transient; clinically significant hepatotoxicity is uncommon but diclofenac is associated with a higher risk of hepatotoxicity among NSAIDs.
- Gastrointestinal ulceration and bleeding: Potentially life-threatening GI bleeding (haematemesis, melaena) from peptic ulcers or erosions. Risk is increased with age, prior GI disease, high doses, long duration, and concurrent anticoagulants, corticosteroids, or other NSAIDs.
- Cardiovascular events: All NSAIDs, including diclofenac, increase the risk of myocardial infarction and stroke, particularly with prolonged use and at higher doses. Diclofenac has one of the highest cardiovascular risk profiles among NSAIDs (comparable to selective COX-2 inhibitors), making careful patient selection essential. It should not be used in patients with established ischaemic heart disease, stroke, or heart failure.
- Acute kidney injury: Particularly in patients who are volume-depleted, elderly, have pre-existing CKD, heart failure, or are on diuretics or RAAS blockers. NSAIDs reduce prostaglandin-dependent renal afferent arteriole dilation, impairing renal perfusion.
- Hepatotoxicity: Clinically significant liver injury, including rare cases of acute liver failure; check liver function if symptoms develop during prolonged use.
- Severe hypersensitivity reactions: Including anaphylaxis, bronchospasm (in aspirin-sensitive asthma), Stevens-Johnson syndrome, and toxic epidermal necrolysis.
- Aseptic meningitis: Very rare; more common in patients with connective tissue disease.
Patients should be informed about the risk of gastrointestinal bleeding and the importance of using the lowest effective dose for the shortest duration.
Drug Interactions
Diclofenac may interact with anticoagulants and other NSAIDs, increasing the risk of bleeding. Always inform your clinician about all medications you are taking.
Additive anticoagulant and antiplatelet effects significantly increase the risk of gastrointestinal and systemic bleeding. Avoid concurrent use unless clinically necessary and closely monitored.
Concurrent use substantially increases the risk of gastrointestinal ulceration and bleeding. Avoid combination of multiple NSAIDs.
Additive gastrointestinal irritation increases the risk of ulcers and bleeding. Use cautiously with gastroprotection where necessary.
NSAIDs may reduce antihypertensive effectiveness and may impair kidney function. Monitor blood pressure and renal function.
Important Warnings
Diclofenac carries one of the highest cardiovascular risk profiles among commonly used NSAIDs — comparable to selective COX-2 inhibitors. It should not be used in patients with established ischaemic heart disease, heart failure, or prior stroke. In patients with cardiovascular risk factors, the shortest effective course at the lowest dose is mandatory.
Diclofenac significantly increases the risk of peptic ulceration and GI bleeding, particularly in older patients, those with prior ulcer disease, and those on concurrent anticoagulants or steroids. A PPI should be co-prescribed in high-risk patients. Seek urgent medical attention if you develop black or tarry stools, vomiting blood, or severe abdominal pain.
NSAIDs reduce renal prostaglandin production, impairing kidney function. In dehydrated patients, those with CKD, heart failure, or taking diuretics and RAAS blockers, diclofenac can cause acute kidney injury. Stay well hydrated; avoid during illnesses causing vomiting or diarrhoea.
Diclofenac should always be used at the lowest effective dose for the shortest necessary duration. It is not a medication for indefinite daily use without regular clinical review.
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Diclofenac FAQs
Diclofenac is generally considered a more potent anti-inflammatory agent than ibuprofen, milligram-for-milligram, and has greater COX-2 selectivity. For moderate-to-severe musculoskeletal pain and inflammatory conditions such as arthritis, diclofenac typically provides superior pain relief compared with standard doses of ibuprofen. However, it also carries a higher cardiovascular risk profile than ibuprofen or naproxen, which influences prescribing decisions.
Topical diclofenac gel (Voltarol Emulgel) achieves therapeutic concentrations in underlying soft tissue while producing very low systemic drug levels, dramatically reducing the risk of systemic adverse effects (GI, cardiovascular, renal). For localised musculoskeletal pain (knee osteoarthritis, soft tissue injuries), topical diclofenac is as effective as oral diclofenac for local pain relief and is recommended as a safer first-line option — particularly in elderly patients.
Diclofenac inhibits COX-1-mediated production of protective prostaglandin E2 in the gastric lining, reducing the mucus layer and bicarbonate secretion that protect against gastric acid erosion. This increases the risk of gastric and duodenal ulcers and bleeding. Co-prescribing a proton pump inhibitor (such as lansoprazole or omeprazole) provides gastric protection and significantly reduces GI ulcer risk in patients taking regular diclofenac, particularly those aged over 65, those with prior ulcer history, or those on concurrent corticosteroids or anticoagulants.
Diclofenac and other NSAIDs promote sodium and water retention via prostaglandin inhibition, raising blood pressure and blunting the effect of most antihypertensive medications. In patients with hypertension, diclofenac should be used with caution, at the lowest effective dose for the shortest duration, with blood pressure monitoring. Paracetamol is the preferred analgesic in hypertensive patients; if an NSAID is required, naproxen may have a slightly more favourable cardiovascular risk profile than diclofenac in this context.