Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine. If you forget to take it If you have pulmonary hypertension and you forget to take a dose of sildenafil, take it as soon as you remember unless it’s nearly time for your next dose. To make sure it’s safe for you, speak to your doctor or pharmacist before taking sildenafil if you: have ever had an allergic reaction to sildenafil or any other medicine are taking medicines called nitrates for chest pain angina have a serious heart or liver problem have recently had a stroke , heart attack or a heart problem — your doctor should carefully check whether your heart can take the additional strain of having sex have low blood pressure hypotension have a rare inherited eye disease, such as retinitis pigmentosa have sickle cell anaemia an abnormality of red blood cells , leukaemia cancer of blood cells or multiple myeloma cancer of bone marrow have a deformity of your penis or Peyronie’s disease curved penis. Using this medicine with any of the following may cause an increased risk of certain side effects but may be unavoidable in some cases. Your doctor may adjust your dose if needed. In some patients, the use of VIAGRA with alpha-blockers can lead to a drop in blood pressure or to fainting medicines called HIV protease inhibitors, such as ritonavir Norvir , indinavir sulfate Crixivan , saquinavir Fortovase or Invirase , or atazanavir sulfate Reyataz some types of oral antifungal medicines, such as ketoconazole Nizoral and itraconazole Sporanox some types of antibiotics, such as clarithromycin Biaxin , telithromycin Ketek , or erythromycin other medicines that treat high blood pressure other medicines or treatments for ED VIAGRA contains sildenafil, which is the same medicine found in another drug called REVATIO. The condition is found in all age groups, including children usually in association with leukemia.Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Priapism warning. If you have an erection that lasts longer than 4 hours, contact your doctor right away. If you have an erection that lasts more than 4 hours, you should get emergency medical care. Go to your nearest emergency room, or call or your local emergency number. Warnings for Viagra.