Diclofenac is a common non-steroidal anti-inflammatory drugs (NSAID) that comes in a variety of formulations. Two of the most common forms are diclofenac sodium and diclofenac potassium (the links refer to the FDA prescribing information of both forms). I have heard several times that the Potassium salt starts to act faster than the sodium salt.
In the second page of both leaflets above there is a table with pharmacokinetic parameters:
- For diclofenac potassium, Tmax (the time it takes to reach maximum concentration after one dose) is 1.0 hour, and the half-life (the time it takes the drug concentration to go down by 50%) is 1.9 hours.
- For diclofenac sodium, Tmax is 2.3 hours, and the half-life (the time it takes the drug concentration to go down by 50%) is 2.3 hours too. This means that for the sodium salt is being "processed" (absorbed, metabolized, eliminated) by the body more slowly than the potassium salt.
One more important point of difference to note is that the sodium salt comes in enteric-coated tablets, while the potassium salt comes in immediate-release tablets.
My question is: Can the difference in "processing time" of both salts by the body stem from the different salts, or is it due to the pharmaceutical formulation (EC tablets compared to IR tablets)?
It might seem at first glance that the difference in formulation should be the answer, but if it is so, why is the cation different between formulations?
I hope anyone can shed light on the mechanism, I was unable to find any useful information that actually addressed this issue.