If you're already taking blood pressure medication to help lower your blood pressure then eating too much garlic could lower your blood pressure too much and eating too much garlic could even cause your blood pressure to get too dangerously low.
Eating garlic in moderation when taking blood pressure medication is relatively safe but always keep a check on your blood pressure readings to be sure that your blood pressure doesn't get too low and if it does you should stop eating the garlic and consult your doctor.