There is no proof of the efficacy of dietary supplements, although many do feel that they are helpful. Surgically, most have neither a positive nor negative effect. Vitamin C is felt by some to be helpful in wound healing and can be safely taken before during and after an operation. Many supplements (effectively all others apart from Vitamin C) should be stopped at least two weeks but preferably one month before an operation, most particularly vitamin E or vitamin E containing compounds since they have an effect on blood clotting, reducing its efficiency and therefore making bleeding and or bruising more likely at surgery.