Many things can affect your ability to achieve an erection, but erectile dysfunction generally boils down to one major issue — lack of blood flow. Either not enough blood comes into the penis or the blood leaves the penis too quickly to achieve or sustain an erection. "When it comes to erectile dysfunction, the healthier the person, the healthier the sexual function," says Dr. David Cozzolino, urologist with Christiana Care Health System and Brandywine Urology Consultants. "The healthier you are, the healthier your erection will be." The usual culprits — heart disease or lack of exercise — should be the first to blame; these will kill your sex quick. But many drugs can also do damage down there. Here are eight substances you might be using that can cause erectile dysfunction.