They’re often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction ED. They are the same and should therefore produce the same outcome whether you opt for the brand-name or the generic. It works for up to 36 hours. When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. These include itraconazole Sporanox, Tolsura , ketoconazole Extina, Ketozole, others and ritonavir Norvir. Medicines that block an enzyme called CYP3A4.These drugs include: nitrate drugs, such as nitroglycerin (Nitro-Dur) alpha-blocker drugs, such as prazosin (Minipress) blood pressure drugs, including amlodipine . sildenafil (Viagra) tadalafil (Cialis) vardenafil (Levitra, Staxyn) Caution: Do not use these ED medications if you take nitrates, such as nitroglycerin or a similar medicine, for chest . How oral medicines differ. Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to .