Vardenafil usually works for 4 to 5 hours. Read more on male sexual health on the Sexual Health Australia site. When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction. Sildenafil may not be safe to take for people with certain medical conditions including: a past history of stroke or heart attack in the last 6 months retinitis pigmentosa an eye disease It can also interact with many other medicines, particularly nitrates used in heart disease , amyl nitrite and blood pressure medicines. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly.Viagra generic form. Viagra is a brand-name medication. If you have any questions about how to store medications or how long they’re good for, talk with your pharmacist. Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects . Teva sildenafil is a generic version of Viagra that treats erectile dysfunction (ED) and heart conditions like pulmonary arterial hypertension (PAH). Viagra is so popular that its name .