Arnica has bee used to treat arthritis,muscle soreness and inflammatory conditions in Europe and is now gaining more wide spread use in the US. A 2007 study by Widrig out of Switzerland compared arnica favorably to NSAIDS for osteoarthritis. By contrast on healthline arnica has a grade of C for pain associated with osteoarthritis. The NIH did not find any difference with respect to bruising versus placebo. I found one study on arnica taken by mouth from a double blinded placebo controlled trial from the Oslo Marathon which demonstrated decreased soreness but no measurable changes in enzymes.
I have not used the oral version for my patients but have had anecdotal success with the topical gel. The oral version at certain
levels can have side effects. Currently the oral is not approved for use by the FDA. I carry a 12% version of the gel made in Germany in my practice and have seen 7% available at drugstores.