I’m using lots of Epsom salts right now. Here’s why.
http://boards.straightdope.com/sdmb/showthread.php?postid=1425457#post1425457
How do Epsom Salts help with aches & pains? You apply it to the outside & it helps the inside. Huh? 
I’d check the Web, but there’s so much BS on health issues online that it’s getting hard to sort things out.
Ben-Gay springs to mind, Tiger Balm, IcyHot… Topical creams (steroids for itching, etc.) all work by being applied on the outside to affect something inside. It’s called absorption.
Since Epsom salts are just magnesium sulfate crystals, I suspect that it is the action of the magnesium that is helping your aches and pains.
Everyone has a webpage and that includes Epsom Salts.
I’m not exactly sure how the stuff works, but I know it does. I had surgery on my foot not too long ago and the podiatrist told me the key to it healing would be if I soaked my toe in epsom salt two hours after after surgery and then three times a day for a week. They didn’t give me a prescription for painkillers and really, I didn’t need one.
That’s not what convinced me of the powers of epsom salt though. When I was in California visiting Shayna, we walked so much that my feet were killing me. That night, I took my tennis shoes off and saw the beginnings of blisters on both feet. You know the type. Raised bumps and looking at it you just know that it’s going to hurt the next day. Imagine three or four of them strategically placed on each foot. Since I was tired and I knew I had to soak my foot, I ran the bath and put epsom salt in the water. I soaked in the tub for an hour and then just fell into the bed and went immediately to sleep.
The next morning, I woke up and my feet weren’t hurting. I looked at the bottom of my feet and there weren’t any blisters either. Nothing. Nada. There wasn’t any indication that I ever had the beginning of a blister. I felt great.
I’m not trying to sound like a commercial for the stuff, but I’m convinced it works. I don’t know how. It just does.
This question came up over at Fathom a while back, check out that discussion here…
http://fff.fathom.org/ubb/Forum7/HTML/000200.html
From this website linked to in the Fathom discussion which talks about its use in treating asthmatics…
http://www.willner.com/References/webref20.htm
“Magnesium works at a cellular level, most likely by displacing calcium. Calcium stimulates one of the primary allergic cells in the body-the mast cell-to burst and release a flood of histamine. It is also necessary for muscle contraction. Magnesium, in contrast, helps stabilize the mast cell and relax muscles, so it functions as both an antiinflammatory nutrient and as a bronchodilator.”
It’s a good additive for gardens also. I use it around my tomato bushes.