Although Vasodilating Drugs, Such as Papaverine and Prostaglandin, Are Effective

Although vasodilating drugs, such as papaverine and prostaglandin, are effective in producing an erection for patients with erectile dysfunction, about 50 to 60 percent of patients discontinue using these drugs.Why do they usually discontinue use?

A) because of the expense of the drug
B) because they become more confident about their sexual abilities
C) because of painful side effects
D) because the effectiveness of the drug decreases with continued use