Rubbing it in: Do topical pain relievers really work?
By Harvard Health Letters
But there are also dozens of topical pain relievers -- creams, ointments, and oils that let us rub and get our pain medication. The over-the-counter products are sometimes grouped into a "muscle rub" section at the drugstore. The cortisone creams and other products for dealing with minor skin irritations are in a different section and aren't covered in this article.
Applying medicine right to where it hurts certainly has a lot of intuitive appeal. And for people whose gastrointestinal tracts don't react well to NSAIDs (a common problem), or who are reluctant to take pills for whatever reason, the topical approach is tempting.
The rub? Lingering doubts about whether these remedies work. And it can be hard to figure out whether a treatment is effective. The bar can be set pretty low: an ointment that provides a soothing sensation for a short while might be said to work by some definitions. And the placebo effect -- benefit that comes from the patient's expectations rather than the treatment itself -- is a major complicating factor in treatment of something as subjective as pain.
On the other hand, there's no question that active medicine can penetrate the skin and get into the body (how much is absorbed is a separate question). And, at least in theory, exposing just a painful area to a medication should mean fewer side effects than taking a pill, which involves gastrointestinal absorption and circulation of the drug in the blood.
Here's a quick rundown of some of the active ingredients in commonly available topical pain relievers:
NSAID gels and ointments are not new, but they're getting a closer look these days. Rofecoxib (Vioxx) and other drugs in the COX-2 class had been positioned as safer, "gut-sparing" alternatives to the oral NSAIDs, but rofecoxib was pulled off the market in 2004, so there's now a gap that the topical NSAIDs might fill. (Celebrex, a different COX-2 inhibitor, is still on the market.)
The gastrointestinal problems (stomach upset, ulcers, bleeding) caused by oral NSAIDs are the result of both direct irritation of the gut's mucosal lining and systemic effects -- chiefly the lowering of prostaglandin levels in the blood, which may reduce the integrity of the gastrointestinal lining. So if an NSAID delivered topically gets into the blood in large amounts and lowers prostaglandin levels, it might very well have a similar side effect profile as one that has been swallowed -- even in the absence of direct contact with gastrointestinal tissue. But from what has been seen so far, gels and ointments result in lower NSAID blood levels than the pill forms of the drugs. For example, blood level from topical diclofenac is about 6 percent of the level that results from the same dose of the drug in oral form.
The research is spotty, but those lower blood levels seem to translate into fewer side effects, aside from local skin irritation. In a study comparing an ibuprofen ointment to ibuprofen pills published in 2008 in the journal BMJ, people in the ointment group suffered fewer side effects than those who took pills.
Fewer side effects don't mean much if topical NSAIDs don't work to ease pain. Indeed, some experts look at the low blood levels and say that topical drugs can't be very effective in such low concentrations, aside from perhaps having some skin-deep, anti-inflammatory effect.
The data from clinical trials are mixed and open to multiple interpretations. A 2004 meta-analysis published in BMJ came to the conclusion that after the first two weeks of use, there was no evidence that topical NSAIDs were any more effective than a placebo. The Medical Letter, a well-regarded newsletter on new therapies, said the diclofenac gel might be modestly effective but also noted that the high placebo response leaves room for doubt. The ibuprofen pill and ointment study published in BMJ ended in a tie in terms of pain relief effectiveness, but researchers noted in their conclusion that one interpretation of the results could be that neither preparation is particularly effective.
The bottom line: NSAID ointments and gels probably are less likely to cause side effects than the oral versions, but there are doubts about how effective they are.
Menthol's familiar cooling sensation is the flip side to capsaicin's burning, although it's not expected to "max out" neurons and cause desensitization like capsaicin. Essentially, it creates a pleasant diversion from pain or other irritations -- a reasonable goal, particularly if it can outlast the pain, but not really a treatment for pain or inflammation. Camphor has a similar effect.
Menthol is an active ingredient in most of the traditional rub-in products, like Absorbine Jr. and Bengay, so in addition to the cooling sensation, the first whiff brings back memories. The only active ingredients in the IcyHot products are menthol and methyl salicylate. It's unclear what would cause a hot sensation.
Like capsaicin, menthol doesn't change the skin's temperature; it creates a cooling sensation by attaching to a certain neuronal receptor. Scientists have found that receptor in cancers, so there's some hope that menthol could be used to make cancer treatment more effective. File this, though, under very preliminary.
The bottom line: Menthol used in topical pain relievers is a harmless substance that causes a pleasing sensation that counteracts pain, but it doesn't influence the underlying cause or inflammation.
Methyl salicylate is a wintergreen-scented compound that's an active ingredient in many over-the-counter pain-relief ointments, including some varieties of Bengay. Scientists have discovered that for the plants that produce it, methyl salicylate seems to be part of a warning system that helps the plant fend off disease. It's one of a group of chemicals known collectively as salicylates because salicylic acid is their shared, root compound. Aspirin -- salicylic acid with an acetyl group attached (thus its formal chemical name, acetylsalicylic acid) -- is the best known of the salicylates. Trolamine salicylate, the active ingredient in Aspercreme, is another salicylate used in topical pain-relief medications.
There's little, if any, rigorous research into methyl salicylate's effectiveness as a pain reliever. On the other hand, there's not much question that once a salicylate compound is absorbed and metabolized into salicylic acid, it has some effect on pain and inflammation, and studies have found that methyl salicylate is well absorbed. A study published in 2008 came to the conclusion that aspirin taken orally and a strong methyl salicylate cream were both effective in making blood platelets less "sticky." Trolamine salicylate hasn't fared so well in absorption research.
The bottom line: Products that contain methyl salicylate might provide some pain relief, but there's no solid proof. Anyone with an aspirin allergy or who is taking blood thinners for cardiovascular disease should consult a doctor before regularly using topical medications that contain salicylates.
Capsaicin is the chemical found in chili peppers that gives them their hot, spicy taste. It's also the active ingredient in several over-the-counter pain products, including Capzasin HP, Sloan's Liniment, and Zostrix. The burning sensation from capsaicin is supposed to do more than just get your mind off the pain, although it does that quite well. In theory, neurons shut down after they've been stimulated by the chemical, so the burning and other unrelated sensations -- including pain -- cease. We say "in theory" because the results from studies testing the low concentrations of capsaicin present in most over-the-counter products (0.075 percent or less) haven't been impressive.
One problem is that people are bothered by the burning sensation, so they don't stick with the treatment. Moreover, capsaicin is poorly absorbed, so the low concentrations don't deliver enough of the chemical to neurons to dependably produce the desensitization that is supposed to make capsaicin more than a distracting irritant. High-dose capsaicin patches have been developed, but they require local or regional anesthesia and therefore would only be appropriate for treatment for severe chronic pain.
The bottom line: The over-the-counter capsaicin products may not be effective for many people.
ACTIVE INGREDIENTS OF SOME OVER-THE-COUNTER TOPICAL PAIN RELIEF PRODUCTS
Product Active ingredients
Absorbine Jr. menthol, 1.27 percent
Aspercreme trolamine salicylate, 10 percent
Bengay (greaseless) menthol, 10 percent; methyl salicylate, 15 percent
Bengay (ultra strength) camphor, 4 percent; menthol, 10 percent; methyl salicylate, 30 percent
Capzasin HP capsaisin, 0.1 percent
Flexall menthol, 16 percent
Freeze It camphor. 0.2 percent; menthol, 3.5 percent
IcyHot Stick menthol, 10 percent; methyl salicylate, 30 percent
Thera-gesic menthol, 1 percent; methyl salicylate, 15 percent
Tiger Balm patch camphor, 80 mg per patch; capsicum extract, 16 mg per patch; menthol, 24 mg per patch
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© 2012, PRESIDENT AND FELLOWS OF HARVARD COLLEGE. ALL RIGHTS RESERVED. DISTRIBUTED BY TRIBUNE MEDIA SERVICES, INC.