As far as I know Arnica is toxic if ingested and you can’t use it on open wounds either. I’ve seen the homeopathic forms that you are supposed to ingest, but I would never use them. My Dad used some homeopathic remedies once that nearly killed him. They do use toxic substances in some of them, so I just avoid anything homeopathic, and stick with herbal treatments. You do need to research things first.
I do use an Arnica gel for pain relief sometimes. It works. Its for external use only.
However, for bruising I use witch hazel. Its a clear liquid, I just put it in a spray bottle and spritz it on and let it dry. You can also use it in a compress. It makes the bruise look worse for a day or two, but it clear up much faster than without the witch hazel. Its also healing for sprains, strains and sore muscles.
Another thing that helps with nasty bruising is comfrey. You can buy comfrey ointment at health food stores. It great for bruises and even helps speed healing with fractures and broken bones. Comfrey is another herb that can be toxic if the wrong variety is taken internally. Comfrey ointment is safe for external use, which is the only way I’ll use it.
I like to spray on the witch hazel and let it dry, then add some comfrey ointment on top of it. When the bruise is not as dark, start using aloe vera gel in place of the witch hazel, or use all three.
I get a lot of bruising because of a bleeding problem, I have several very dark bruises from shots right now, and they are all healing nicely now. Once I start this treatment they really heal fast, compared to the usual.