least four hours apart. [ 22 ] Contraindications Contraindications  include: [ citation needed ] When taking nitric oxide donors, organic nitrites and nitrates, such as  glyceryl trinitrate  (nitroglycerin),  sodium nitroprusside ,  amyl nitrite  (" poppers ") [ 23 ] In men for whom  sexual intercourse  is inadvisable due to  cardiovascular  risk factors