Capsaicin – Why Doesn’t It Work As Needed (PRN)?
A 76-year-old male has been having some knee pain due to osteoarthritis. He has tried acetaminophen and ibuprofen and didn’t find much help with those. He finds a topical agent called capsaicin. He tries it PRN (as needed) for a couple of days and doesn’t really notice any changes. He stops taking it.
This is a scenario that I have seen before. It is very important to educate patients about the appropriate use of topical capsaicin. Let’s discuss the likely reason as to why the drug didn’t help this patient.
Capsaicin’s mechanism of action likely involves the depletion of substance P. Substance P is well known to play an important role in sending pain signals to the CNS. Capsaicin can facilitate the release of substance P and deplete the target area(s) (knee in this case) stores of the compound. Over time, this can lead to lower amounts of substance P in the target area and help prevent the body from restoring its normal supply. This only occurs with routine use of the medication.
Because of the drug’s mechanism of action and way that it reduces pain impulses, the onset of action is approximately 2-4 weeks.
When you compare this to another topical agent like diclofenac, you can understand the difference and why patients might think that once they start taking it, they should feel relief. Diclofenac gel can start providing relief through its prostaglandin lowering effect within hours of application.
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