Smith1; Mary Babos2. For erectile dysfunction, it’s best to take sildenafil on an empty stomach. Angina severe chest pain , unstable or Arrhythmia irregular heartbeat, within the last 6 months or Blood vessel problems eg, aortic stenosis, idiopathic subaortic stenosis or Heart attack within the last 6 months or Heart failure or Hypertension high blood pressure or Hypotension low blood pressure or Retinal disorders eye problem or Retinitis pigmentosa an inherited eye disorder or Stroke within the last 6 months —Should not be used in patients with these conditions. If you do not have a plastic syringe or spoon, ask your pharmacist for one. Dosing The dose of this medicine will be different for different patients. Use protective measures and ask your doctor if you have any questions about this. Sildenafil usually should not be taken more than once every 24 hours.Bookshelf ID: NBK PMID: Sildenafil is a medication used in the management and treatment of erectile dysfunction and pulmonary arterial hypertension. It is in the phosphodiesterase-5 inhibitor (PDE5-I) class of medications. This activity describes the indications, actions, adverse effects, and contraindications of .  · If you take sildenafil for pulmonary arterial hypertension, do not take Viagra® or other PDE5 inhibitors, including tadalafil (Cialis®) or vardenafil (Levitra®). Viagra® also contains sildenafil. If you take too much sildenafil or take it together with these medicines, the chance for side effects will be higher. Taking a counterfeit drug could put you at risk for harmful health effects. For information on making sildenafil more affordable, see the “Ways to reduce long-term .