I have used arnica topically in the past but hadn't considered it for this surgery.
I found this info though:
Side Effects of Arnica
Arnica has been seen to be safe when used in diluted or homeopathic doses for a maximum of two weeks under constant supervision of a doctor or health care professional. However, at extremely higher doses, arnica montana can lead to very serious side effects and even death. Possible side effects of high arnica dosage include stomach discomfort, nausea and vomiting, as well as liver and kidney damage and organ failure.
Skin rashes, lesions of the mouth and eczema might occur in some people as a result of allergies. Topical arnica should never be used on open wounds or anywhere near the eyes and mouth. There have also been reported cases of mouth ulcers caused by arnica-containing mouthwash.
Other less probable side effects of increased arnica doses taken orally include irregular heart rhythm, high blood pressure, rapid heartbeat and heat failure. Theoretically, arnica can increase the possibility of bleeding. If you have a history of diabetes or have used anticoagulant or antiplatelet drugs, always consult a doctor before taking arnica, even at homeopathic doses.
Arnica and Surgery
Several research studies have suggested that homeopathic doses of arnica can relieve pain after surgery. It has also been seen to reduce hematoma and bruising after surgery and promotes faster post-operative recovery.