Taking Advantage of the Peripheral Opioid Receptor
by Forest Tennant, MD, DrPH
 Figure 1. Illustration of the inflammatory pain site.
History may arguably show the discovery of the peripheral opioid receptor to be the
most important clinical discovery in pain treatment in the past 20 years.1-6
This critical finding gives the practitioner and patient many therapeutic options other
than oral opioids. About a year ago in these pages, I encouraged the use of topical
morphine. Since that note, I have received much correspondence about the use of other
topical opioids and I too have been experimenting with opioids other than morphine. In
this effort, I have even encountered some patients who use topical morphine in enough
quantity to produce a measurable blood level and some patients even report analgesic
effects distant to the local, applied area. Of particular benefit has been the use of
topical opioids for knee and spine pain.
The Inflammatory Pain Site
Shown here is an artist's rendition of the anatomical pain site (See Figure 1). It is
fundamentally a wound in that it contains inflammatory compounds, sequestered electrical
charges, and opioid receptors which are now known to propagate in inflamed tissue.7
The natural function of opioid receptors in inflammatory pain sites is to presumably
attract the body's natural endorphin compounds for pain relief and immune enhancement.
Re-tained or sequestered electrical charges in the pain site generate pain. The removal or
displacement of sequestered electrical charges by various electromagnetic means including
such simple measures as copper or magnets and various electromagnetic
instruments—including electric currents, ultrasound, radiofrequency, infrared, and
laser—are generally effective in reducing pain when electromagnetic measures are
concomitantly used with topical opioids.
Which Opioids?
Not all opioids provide topical pain relief. The opioids most commonly prescribed by the
author are: (1) morphine; (2) hydromorphone; and (3) oxycodone. Although not officially
classified as an opioid, carisoprodol, the notorious and abusable Soma«,
produces considerable analgesia when topically applied. Some patients use methadone
topically and find it effective. There are some opioids that are inert on the skin
surface. Hydrocodone, codeine, and tramadol are pro-drugs which require liver metabolism
to convert them to active compounds. Fentanyl is extremely soluble and dissolves quickly
through the skin making it a very effective systemic, but poor topical opioid. Morphine,
hydromorphone, and oxycodone are relatively insoluble and act directly on opioid receptors
without requiring further metabolism.
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— April 2010
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