The main side effects of a long erection is oxygen deprivation. The blood trapped in the penis is deprived of oxygen. When an erection lasts for too long usually more than four hours, it is a problem. This lack of oxygen can begin to damage or destroy tissues in the penis. Untreated priapism can cause erectile dysfunction. Consult the doctor immediately, if If the condition is not treated immediately, it can lead to scarring and permanent erectile dysfunction.