Best to stop all diet pills, aspirin and herbal supplements 2 weeks prior to surgery. Diet pills have affects and can have side effects like blood pressure issues
I give my patients a list of medications that I would like for them to stop at least 2 weeks prior to surgery because many medications increase your risk of bleeding. Diet pills are not regulated in the US and there is not a good way to determine exactly what you are taking. So I would say review your medications with your doctor and stop anything that will increase your bleeding risk.