My friend brought me a combination of belladonna and arnica. What are its uses?

It comes in a small jar and is the consistency of Vicks Vaporub. My friend said it would be good to rub into my skin over arthritic joints to dull the pain. Is she correct? What other uses would it have? What side effects could it have? (P.S. She got it in a Mexican homeopathic market.) Thanks for your help.