It's not just about rubbing--topical capsaicin and topical salicylates may be useful as adjuvants to conventional pain treatment

doi:10.1136/bmj.328.7446.998

BMJ 2004;328:998 (24 April), doi:10.1136/bmj.328.7446.998

Commentary

It's not just about rubbing—topical capsaicin and topical salicylates may be useful as adjuvants to conventional pain treatment

Martin R Tramèr, consultant anaesthetist¹

¹ Division of Anaesthesiology, Geneva University Hospitals, CH-1211 Geneva 14, Switzerland martin.tramer{at}hcuge.ch

Drugs can be injected (subcutaneously, intramuscularly, intravenously,intrathecally, epidurally); given by mouth (orally, sublingually),intranasally, or rectally; or inhaled. They can also be appliedto the skin. The transdermal method is suitable for certainlipid soluble drugs and produces a steady rate of delivery forup to three days. Cutaneous administration is used when a localeffect on the skin is required. Drugs may also be applied tothe skin to achieve close proximity to the bones or muscleswithout flooding the organism; this is the case when, for instance,an analgesic cream is applied to a painful knee.

The two reviews by Mason and coworkers (Oxford Pain ResearchGroup) provide evidence that analgesic creams and ointmentsmay be useful for treating some acute and chronic pain.^1
2 Topicalcapsaicin, for instance, shows some efficacy in neuropathicpain. Topical salicylates work in strains, sprains, and sportinjuries; the same has been shown for topical non-steroidalanti-inflammatory drugs.³

Several things need to be considered when putting these datainto a clinical context. Firstly, each of these remedies hasa biological basis for an analgesic effect, and this supportstheir usefulness. Consequently this is not just about rubbing;it is about molecules that have an effect on cutaneous nociceptors(capsaicin) and tissue cyclooxygenase (aspirin and non-steroidalanti-inflammatory drugs). Secondly, that a drug is applied tothe skin does not necessarily have fewer adverse effects. Withtopical capsaicin, one third of the patients are likely to experiencesome local skin irritation; one in 10 may even stop treatment.Thirdly, none of the topical analgesics is universally efficacious.Putting this together, we may define some pragmatic clinicalguidelines. For instance, topical capsaicin is unlikely to bea first choice treatment for neuropathic pain—there issimply not enough analgesia and there is too much harm. However,it may be regarded as an adjuvant to standard treatment forneuropathic pain with conventional or unconventional analgesics,or it may serve as a last resource when everything else hasfailed. With topical salicylates and non-steroidal anti-inflammatorydrugs there are few local and almost no systemic adverse effects.These creams may be used as a first line treatment in, for instance,sport injuries—especially considering their availabilityover the counter.

The question then is why topical analgesics are popular amongpatients but do not have a good reputation among doctors. Onereason may be the apparent unreliability of the existing evidencesupporting their usefulness; indeed, there are not many relevantpublished trials, and most are of low quality, questionablevalidity, and limited size. As a consequence, many doctors arenot convinced that the creams work. This makes systematic reviewsthat use a stringent methodological approach, such as thoseby Mason and coworkers, so valuable. As a consequence of thisapproach, the results may be less advantageous than in other,less rigorous studies (the numbers needed to treat are higher),but at least there is assurance that the evidence is viewedin the most appropriate light and that the results can be trusted.

Funding: None.

Competing interest: None declared.

References

Mason L, Moore RA, Derry S, Edwards JE, McQuay J. Systematic review of topical capsaicin for the treatment of chronic pain. BMJ 2004;328: 991-4.[Abstract/Free Full Text]
Mason L, Moore RA, Edwards JE, McQuay HJ, Derry S, Wiffen PJ. Systematic review of topical rubefacients containing salicylates for the treatment of acute and chronic pain. BMJ 2004;328: 995-7.[Abstract/Free Full Text]
Moore RA, Tramèr MR, Carroll D, Wiffen PJ, McQuay HJ. Quantitative systematic review of topically applied non-steroidal anti-inflammatory drugs. BMJ 1998;316: 333-8.[Abstract/Free Full Text]