Dear all 3 (!) patients who have asked me this:
In short, I do not know! The first time this came up in a patient encounter, I laughed aloud at the absurdity of it. Putting a bar of soap under the covers, not even necessarily in contact with the body, relieves pain and cramping? Was I understanding this correctly?
As an oncology fellow, the brave patients I see are no strangers to pain. They are often well-educated, well-informed individuals on top of both the latest clinical trials, as well as integrative therapies, such as herbs, supplements, and acupuncture. They teach me so much about the gift of life, but also about the innovative and effective ways of overcoming their corporeal struggles. I value their stories, as their unique methods of combating cancer and its symptoms along the arduous path of controlled, clinical studies can be vital to the next patient, who might be facing a similar plight.
Still skeptical after my third patient mentioned relief from this method, I took to Pubmed (the resource with over 27 million biomedical publications) to see if any studies had been done to investigate this sudsy mystery. Lo and behold, a case report had been published from the Anesthesiology Department in Wisconsin about using a soap-scented oil skin patch for effectively treating pain associated with fibromyalgia . Based on the recommendation of a Dr. Gott, who successfully treated patients by advising them to sleep with a bar of soap between their legs and published his experience in a popular newspaper column, this study first involved creating a skin patch made of crushed bar soap to relieve leg muscle cramping and pain in 14 patients . The patch worked. Expanding on this idea, Dr. Ough, the lead investigator, hypothesized that it was the scent of the oil itself, applied directly to the skin, that was responsible for the pain-relieving and muscle-relaxant properties of the skin patch.
This would make sense, given my patients' reports on relief from just having the soap in the bed and not even touching their legs, as the only volatile property of the soap that could diffuse through the sheets is its scent. Dr. Ough then assembled the skin patches with a soap-scented oil (SSO), rather than bar soap itself, and took to treating the 14 patients suffering with severe pain.
What happened? All patients reported initial pain relief within 1 hour of application, with 3 patients reporting nearly complete pain relief (lasting between 18 to 30 hours). Their sleep was also more restful. The patch relieved pain from muscle cramps, knots, and trigger point pain, in addition to smooth muscle spasms, relieving menstrual cramps, intestinal cramps, and the pain from a kidney stone. Shockingly, this would represent a new and unique method of medicinal delivery, as scent is seemingly absorbed through skin and not through the nose!
Broadening my investigation to the wild, wild, world wide web, it seems the history of this hypothesis dates back to several years ago, when the advice columnist Ann Landers raised a provocative questions in her column: does putting soap the foot of the bed cure night-time leg cramps? The medical community consensus was no, but anecdotal reports of improved symptoms persisted.
From peoplespharmacy.com , 219 people rated this modality 3.5 stars out of 5. The author noted the following :
"What People Report:
- It seems to work for many people. Soap in the bed appears to alleviate nocturnal leg cramps.
- Relief is immediate and sustained.
- Some people report that soap does not work. It appears either to work consistently and well or not at all. There are few cases of partial success.
- After a few months, a bar of soap is no longer effective for preventing cramps. It must be replaced. Old soap can be rejuvenated by scoring or shaving it to produce fresh surfaces.
- Some subjects have placed the soap between the sheets, and some have placed it under the bottom sheet. Either or both of these methods work.
- Some subjects report that direct physical contact between the subject and the soap is desirable, but few claim it is essential."
Since direct contact did not seem essential, a volatile compound needs to pass through the air to the body, and the only possible source would be the small molecules of fragrance. Also, as soap ages, it becomes less porous, which would explain why new soap bars work better than old ones.
Moreover, the esters and oils making up fragrances such as lavender oil, are vasodilators [3,4] Vasodilators work by enlarging blood vessels, the same way nitroglycerin treats angina chest-pain.
Bottom line: Given the case reports and plausible mechanism of action, there just might be some solid science beneath this slippery solution of a cure for pain. Allergies aside, the risks of such a treatment seem minimal in the face of refractory pain, with a lot of relief to gain. Worse case, at least your bed sheets will smell good!
 Ough YD. Soap-scented oil skin patch in the treatment of fibromyalgia: A case series. Journal of multidisciplinary healthcare. 2008;1:59-62.
 Shimada K, et al. Aromatherapy alleviates endothelial dysfunction of medical staff after night-shift work: preliminary observations. Hypertens Res. 2011 Feb;34(2):264-7. doi: 10.1038/hr.2010.228. Epub 2010 Nov 25.
 Koto J, et al. Linalyl acetate as a major ingredient of lavendar essential oil relaxes the rabbit vascular smooth muscle through dephosphorylation of myosin light chain. J Cardiovasc Pharmacol. 2006 Jul;48(1):850-6.