On 500 mg aspirin daily, to prevent a heart attack, cad patient. is it safe?

500 mg ASA daily is a large dose. As you identify as a coronary artery disease patient you should also already be under the care of a specialist Cardiologist. Ask him/her your question, especially in light of recent study that shows there may be greater mobility than benefit from daily 81 mg ASA.

The Aspirin typical range for cardiovascular events prevention is 81-162 mg/day and an evaluation for risk for bleeding is required. Your primary care doc and/or cardio would be best to advise in proper clinical context.