Firstly, forgive that truly horrific use of French in the title. I'm truly sorry.
Okay, so today I've been having a more in-depth look at the evidence that Salonpas works. And, as predicted, the bottom line is that there seems to be very little. I'm going to tell you where I've looked, in what order, and what I've found and you can see if you agree with this conclusion.
Firstly, the manufacturer's info, the Summary of Product Characteristics. This tells me how it works- it contains methylsalicylate which is converted into salicylic acid (an anti-inflammatory which also irritates the skin), and levomenthol, which causes a local cooling effect and also causes irritation. You might think including ingredients that cause irritation might be weird, but in fact this is partly how most topical rubs and sprays-and even just plain heat- works. Such things are called rubefacients. The irritation basically detracts from the pain and is supposed to distract you from noticing it- replacing one type of pain with another, almost.
So now we know that we can try to find some proper evidence that this particular product works. So I've done a Medline and Embase search, freetext searching using the brand name. the results? zip. not a sausage. So I tried searching methylsalicylate, and again I found nothing of use. Hmmm.
Eventually I've managed to dig out one study by the manufacturers of Salonpas. I can only get it in abstract form, so I can't really appraise it properly, but its pretty small (n=208) and is limited to patients with mild to moderate muscle strain. Now, if you've ever done any exercise ever, you'll probably know that muscle strain is a self-limiting condition anyway which may well get better itself after a few hours. I also don't know from the abstract whether they controlled the amount of activity in the patients- as you'll know, keeping moving can really help with muscle strain, whereas keeping still can mean you seize up and hurt more. I'm wondering about the blinding of the study as well: what did the placebo contain, because with levomenthol in the active patch it would be easy to distinguish by smell which was which. The results do seem to find a significant difference in pain, but are mainly reported as a per-protocol analysis, so don't account for any patients who have dropped out of the study.
So one study suggests some benefit, but may have some flaws that mean we can't quite accept the results yet.
What about topical rubefacient and anti inflammatory agents as a whole? Well, a Cochrane review in 2004 found that the available studies are limited by poor design, that rubefacients are pretty rubbish in acute pain, but they might work for 1 in 6 patients above those that respond due to placebo effect with chronic pain. The same review reveals that topical NSAIDs might benefit 1 in 3 patients with chronic pain.
Part of the reason these topical preparations work for some people actually has little to do with the drug content of them and a lot to do with application technique: rubbing the product into the area itself can increase blood flow and soothe the area. These patches eliminate that effect, which I think is a really problem in justifying their use. The side effects look pretty innocuous for Salonpas, but I would postulate that having irritants in constant contact with the skin for 8-12 hours, rather than acutely rubbing a small amount of cream into the area, would lead to more local skin irritation.
I have emailed the manufacturers to see if they have any more evidence in their favour. I guess we'll have to wait and see what they come back with, but in the meantime: Will I be recommending this product to patients? Definitely not. There's limited evidence that they work, and no evidence to suggest they are better than the existing (cheaper) products on the market. In short, its yet another gimmick to fill up the shelves in a pharmacy and yet another way to get people to part with their money with little justification. Would you agree?