The amount of medicine that you take depends on the strength of the medicine. Especially tell your healthcare provider if you take any of the following: medicines called nitrates medicines called guanylate cyclase stimulators such as Adempas riociguat medicines called alpha-blockers, such as Hytrin terazosin HCl , Flomax tamsulosin HCl , Cardura doxazosin mesylate , Minipress prazosin HCl , Uroxatral alfuzosin HCl , Jalyn dutasteride and tamsulosin HCl , or Rapaflo silodosin. This will increase the supply of blood to the lungs and reduce the workload of the heart. If too much is used, the chance of side effects is increased. These medicines all work well for ED. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. From Mayo Clinic to your inbox Sign up for free and stay up to date on research advancements, health tips, current health topics, and expertise on managing health.Take sildenafil at least 30 minutes, but no more than 4 hours, before sexual activity (1 hour before is the most effective). Do not take more than once daily. The dosage is based on your. Sildenafil should not be taken with other PDE-5 inhibitors. The dosage may need to be reduced in people with kidney or liver disease. More research is needed to establish if sildenafil has a role in sexual dysfunction in women. Sildenafil offers no protection against sexually transmitted diseases. AdMany Americans pay far too much for their prescriptions. We’re here to help. Compare pharmacy prices in your area, get the free coupon, & see how much you can save!”This free app will find the best local deals.” –