Diclofenac
Third line non-steroidal anti-inflammatory drug (NSAID)
Mechanism of action: A phenylacetic acid derivative non-steroidal anti-inflammatory drug (NSAID). It is a non-selective inhibitor of cyclooxygenases COX-1 and COX-2, thus results in a decrease in the synthesis of prostaglandins which are involved in pain and inflammatory processes.
Presentation: tablet, suppository, solution for injection, topical gel
Suggested dose: 50mg three times a day for adults over 50kg with normal hepatic and renal function
Co-prescribe proton pump inhibitor (PPI) cover.
Oral absorption: High bioavailability, peak plasma concentration at 1.5-2 hours
Protein binding: more than 99%
Metabolism: Diclofenac is metabolized in the liver by cytochrome P450.
Elimination: The metabolites of diclofenac and a small amount of unchanged drug are mainly excreted in the urine (70%), with around 30% excreted in bile
Half-life: approx. 2 hours
Significant interactions (see the BNF for more interactions):
- Aspirin: NSAIDs competitively inhibit aspirin binding to platelets so antagonise the cardioprotective effect of low-dose aspirin
- Lithium: NSAIDs increase the serum lithium level and reduce lithium clearance, risking acute lithium intoxication
- Anti-hypertensives: NSAIDs compromise the efficacy of anti-hypertensive drugs such as ß-adrenergic blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers and diuretics
- Methotrexate: toxic interaction between diclofenac and methotrexate
Further prescribing information (side effects, contraindications, interactions):