Tell your doctor if you have or ever have had a slow or irregular heartbeat, a recent heart attack, an aortic aneurysm swelling of the large artery that carries blood from the heart to the body, high blood pressure, peripheral vascular disease poor circulation in the blood vessels, Marfan syndrome a genetic condition that can affect the heart, eyes, blood vessels and bones, Ehlers- Danlos syndrome a genetic condition that can affect skin, joints, or blood vessels, or if you have a low level of potassium or magnesium in your blood. doxycycline for fish Frederick, MD U.