Alprostadil is a type of medicine that causes blood vessels to expand, increasing blood flow throughout the body. By increasing blood flow to the penis, alprostadil helps facilitate an erection.lprostadil delivered via a suppository is not as effective as the injection -- it produces an erection in approximately 30%-40% of men with ED. Alprostadil usually begins to work in about five to 20 minutes. Intercourse should be attempted within 10 to 30 minutes after using the medication.