Diclofenac: A potent NSAID for oral and topical use

What it is

Diclofenac is an NSAID from the arylacetic acid group, available in many forms: oral (tablets, fast-release soft capsules), topical (gel, patch) and injectable.

In Spain, oral diclofenac requires a prescription. The topical gel is available without a prescription. It is one of the most potent NSAIDs in its class, with greater COX-2 preference than ibuprofen, although it is not as selective as celecoxib.

Preferential COX-2 inhibition with less COX-1 effect

Diclofenac has a somewhat different COX inhibition profile from ibuprofen:

It inhibits COX-1 and COX-2, but with a greater preference for COX-2 than ibuprofen.
Less COX-1 inhibition → in theory, lower gastric risk than other NSAIDs at equivalent doses.
However, clinical studies show gastrointestinal risk similar to or greater than ibuprofen in practice.
Higher cardiovascular risk than ibuprofen and naproxen: similar to that of the selective COX-2 inhibitors.
The topical form acts locally with minimal systemic absorption: a much better safety profile.

At the counter

When to recommend it

Topical gel (no prescription): localised muscle pain, bruises, sprains, tendinitis, joint pain
The topical form is the first option when pain is localised: comparable efficacy to the oral form with minimal systemic effects
Oral (on prescription): moderate to severe joint pain, dysmenorrhoea, post-operative pain
In inflammatory pain where ibuprofen has not been enough

When not to

Oral without a prescription: in Spain it requires one
History of cardiovascular disease (heart attack, stroke): it increases cardiovascular risk
Moderate to severe heart failure
Pregnancy (avoid from week 20; contraindicated in the third trimester) and breastfeeding
Moderate to severe kidney failure
Do not combine with other NSAIDs or with anticoagulants without medical supervision

What to say at the counter. At the counter: diclofenac gel is an excellent option for localised musculoskeletal pain. It acts directly on the area, with local concentrations far above systemic ones and minimal gastric risk. Ideal when the patient has contraindications to oral NSAIDs.

Warnings

Important CARDIOVASCULAR RISK: oral diclofenac has the highest cardiovascular risk of the commonly used NSAIDs, comparable to the selective COX-2 inhibitors. Avoid in patients with established cardiovascular disease.
Caution LIVER TOXICITY: diclofenac carries a higher risk of liver toxicity than other NSAIDs. Raised liver enzymes and even fulminant hepatitis (rare but serious) have been described. Avoid in liver disease.
Caution INTERACTION WITH LITHIUM: diclofenac reduces the renal excretion of lithium and can cause lithium toxicity.
Caution TOPICAL GEL: although systemic absorption is low, over large areas or under occlusion it can be significant. Do not apply to broken skin or mucous membranes.
Worth knowing PHOTOSENSITIVITY: topical use can increase the sun sensitivity of the treated area. Protect it from the sun after applying the gel.

Pharmacokinetics

Absorption: Oral: rapid and complete absorption (almost 100%), but intense hepatic first-pass metabolism → bioavailability of 50–60%. Fast-release soft capsules act sooner. Peak plasma level: 1–2 hours.
Distribution: High plasma protein binding (>99%). It distributes well into inflamed tissues, synovial fluid (useful in arthritis) and muscle.
Metabolism: Intense hepatic metabolism by CYP2C9 (mainly) and CYP3A4. It produces several metabolites, some active. Caution with CYP2C9 inhibitors.
Elimination: Renal (65%) and biliary (35%). Half-life: 1–2 hours. Duration of clinical effect: 6–8 hours (standard formulations) or 12–24 hours (prolonged release).

Topical diclofenac vs oral diclofenac

Aspect	Topical diclofenac	Oral diclofenac
Systemic absorption	Very low (3–10% of the dose)	High (~50–60% after first pass)
Local concentration	Very high in subcutaneous tissues	Depends on systemic distribution
Gastric risk	Minimal	Moderate
Cardiovascular risk	Minimal	Increased
Indication	Localised musculoskeletal pain	Moderate to severe widespread or joint pain
Available without prescription	Yes (gel in Spain)	No (prescription required)

For localised pain, the topical gel is the first choice: just as effective in the area, with a tiny fraction of the systemic risks. The oral form is kept for more widespread pain or when the topical route is not enough.

Self-assessment

Three questions. When you check your answers you will see the explanation for each one.

A 70-year-old patient with a history of heart attack asks for oral diclofenac for knee pain. What do you do? Give it, diclofenac is safe Offer topical diclofenac gel as a safer alternative, and recommend they see their doctor about managing the pain Give them ibuprofen, which is safer
What is the main advantage of diclofenac gel over oral NSAIDs? It is more potent than oral diclofenac It achieves high local concentrations with minimal systemic absorption, reducing general side effects It acts faster than any NSAID
Why does diclofenac carry a higher cardiovascular risk than ibuprofen or naproxen? Because its standard dose is higher Because it is eliminated through the heart Because its greater COX-2 preference reduces protective prostacyclin more, without balancing it with TXA2

Training content. It does not replace the summary of product characteristics or clinical judgement.

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