Diclofenac salts: potassium or sodium?

A. Oral formulations of diclofenac are formulated as either the sodium or potassium salt. The sodium salt is found in the original prescription-only formulation of Voltarol (enteric coated) 25mg and 50mg tablets and its generic equivalents that account for most oral diclofenac preparations currently used in the UK. The potassium salt is contained in POM Voltarol Rapid 25mg and 50mg products and their generic equivalents; as well as in the over-the-counter products such as Voltarol Joint Pain, Voltarol Pain-eze, and Actavis’s Double Action Pain Relief tablets, which are also marketed as Boots Diclofenac Potassium Tablets. All these OTC diclofenac potassium products contain 12.5mg per tablet and share the adult dose regimen of two tablets initially, followed by one or two tablets every four to six hours as needed, with no more than six tablets (ie, 75mg) to be taken in any 24-hour period. They are indicated for short-term relief of headache, dental pain, period pain, rheumatic pain, muscular pain and backache and the symptoms of colds and influenza, including fever; with a recommendation not to exceed three days’ treatment. The OTC tablets can be used by people over 14 years of age.

Onset of action

The most significant difference between the potassium and sodium salts is in the rate of absorption and the related onset of action. Following ingestion of diclofenac potassium tablets, mean peak plasma diclofenac concentrations are attained after 30 to 54 minutes compared with two to four hours after ingestion of oral diclofenac sodium tablets. (Peak levels of the drug occur in synovial fluid around two to four hours after the peak in plasma levels and this is relevant where it is being used for musculoskeletal problems.)

For patients taking regular twice or three times a day doses of diclofenac, the faster onset of the potassium salt is unlikely to be clinically relevant, however for intermittent use in a variety of acute pain scenarios, this is a potentially useful feature and fits well with the approved indications for OTC use.

Potassium control

Patients who have been advised to restrict their potassium intake may have concerns regarding the use of a diclofenac potassium product. In addition, patient information leaflets for these products advise those on a controlled potassium diet to consult their GP or pharmacist. If such concerns arise, pharmacists can reassure patients that the potassium content of products such as Voltarol Pain-Eze or Joint Pain is extremely low at 0.03734mmols (1.46mg) per 12.5mg tablet — in comparison the potassium content of a single banana is typically between 300 and 600mg. Nonetheless, it is worth recognising that patients watching their potassium intake may have advanced renal disease or be taking angiotensin-converting enzyme inhibitors, angiotensin receptors blockers or potassium sparing diuretics and in such patients it would be wise to avoid all systemic non-steroidal anti-inflammatory drugs, not just diclofenac potassium products.

Dosage regimen

The maximum daily dose of diclofenac approved for OTC use is 75mg. Although this may be a lower dose than what the patient has been prescribed previously, the pharmacist should not recommend that the patient exceeds 75mg/day without medical advice. For a number of years, best practice in the use of systemic NSAIDs has been to follow the principle of using the lowest effective dose for the shortest time necessary to control symptoms. On this basis, the pharmacist should advise the woman to ensure that she returns to her GP to discuss the appropriateness of continuing with diclofenac, and not to continue to use the OTC product in lieu of medical review.

Through further discussion the pharmacist may identify potential opportunities to help improve the woman’s back pain, for example, by supplementing the diclofenac with regular paracetamol and non-pharmacological management strategies such as keeping active and thermotherapy.