Hello. If you are taking blood thinning medication, please check with your Doctor or pharmacist before following this advice.
Arnica, whether in the form of homeopathic tablets or applied topically isn't regarded as something that will prevent bruising occurring.
For that you'd be better trying some extra vitamin C with the associated bioflavonoids, called hesperidin and rutin. We have a nice, naturally-sourced product called 'Nature C' which comes straight from fruit juices, so is easily absorbed.
Arnica gel and tablets are certainly appropriate to treat the bruises that may be there, though. Neither form has any medical interactions to worry about.