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Thu 25 Dec 2008 04:00 AM

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Rub the pain away

Pain-relieving creams and ointments can get the medicine right to where it hurts, and the smell is often familiar and soothing. But do they work?

Pain-relieving creams and ointments can get the medicine right to where it hurts, and the smell is often familiar and soothing. But do they work?

When something like a knee hurts, there's a natural tendency to rub it. And if it really hurts, most of us will think about popping a pain-relieving pill of some kind - acetaminophen (Tylenol), or perhaps one of the nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin, ibuprofen, or naproxen.

But there are also dozens of topical pain relievers - creams, ointments, and oils that we can rub into the painful area and also get pain medication from. These products are sometimes grouped into a "muscle rub" section at the pharmacy.

Anyone taking blood thinners for cardiovascular disease should consult a doctor before using topical medications that contain salicylates.

Applying medicine right to where it hurts certainly has a lot of appeal. And for people whose gastrointestinal tracts don't react well to NSAIDs - a common problem - or who are reluctant to take pills, the topical approach is tempting.

There's no question that active medicine can penetrate the skin and get into the body. 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:

NSAIDs

Anti-inflammatory gels and ointments are not new, but they're getting a closer inspection these days.

Rofecoxib (Vioxx) and other drugs in this class had been positioned as safer, "gut-sparing" alternatives to the oral NSAIDs, but rofecoxib was pulled off the market in 2004 (Celebrex, a different COX-2 inhibitor, is still on the market).

The FDA has approved a gel form of an NSAID called diclofenac for osteoarthritis and there are diclofenac patches. Several ibuprofen creams are also available.

The gastrointestinal problems (stomach upset, ulcers, bleeding) caused by oral NSAIDs are the result of direct irritation of the gut's mucosal lining and the lowering of prostaglandin levels in the blood, which may weaken the gastrointestinal lining.

So if an NSAID delivered through the skin gets into the blood in large amounts and lowers prostaglandin levels, it might cause similar side effects to an oral NSAID. But from what has been seen so far, gels and ointments result in lower NSAID blood levels than the pill forms of the drugs.

The research is spotty, but those lower blood levels seem to translate into fewer side effects, aside from skin irritation. In a study comparing an ibuprofen ointment to ibuprofen pills published in 2008 in the British Medical Journal (BMJ), people in the ointment group suffered fewer side effects than those who took pills.

But fewer side effects don't mean much if topical NSAIDs don't 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 concluded that there was no evidence that topical NSAIDs were any more effective than a placebo (sugar pill).

The Medical Letter, a respected newsletter on new therapies, said the diclofenac gel might be modestly effective but 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, but researchers concluded that one could say 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

The familiar, cooling sensation menthol provides is the flip side to capsaicin's burning, although it's not expected to "max out" neurons and cause desensitisation 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, so in addition to the cooling sensation, the first whiff brings back memories.

Like capsaicin, menthol doesn't change the skin's temperature. It creates a cooling sensation by attaching to a certain neuronal receptor.

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 of inflammation.

Methyl salicylate

This is a wintergreen-scented compound that's an active ingredient in many pain-relief ointments.

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. Aspirin 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 rigorous research into methyl salicylate's effectiveness as a pain reliever. But there isn't much question that once a salicylate compound is absorbed and metabolised into salicylic acid, it has some effect on pain and inflammation.

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

This is the chemical found in chilli peppers that gives them their hot, spicy taste. It's also the active ingredient in several pain products. The burning sensation from capsaicin is supposed to do more than just get your mind off the pain.

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 products haven't been impressive.

One problem is that people are bothered by the burning sensation, so they don't stick with the treatment.

In addition, capsaicin is poorly absorbed, so the low concentrations don't deliver enough of the chemical to neurons to dependably produce the desensitisation that is supposed to make capsaicin more than a distracting irritant.

High-dose capsaicin patches have been developed, but they require local or regional anaesthesia and therefore would only be appropriate for treatment for severe chronic pain.

The bottom line: The capsaicin products may not be effective for many people.

Source: Harvard Health Publications. Copyright 2008 President and Fellows of Harvard College. All rights reserved.

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