Dizziness, lightheadedness, headache, flushing, stomach upset, nosebleeds, trouble sleeping, or swollen hands/ankles/feet (edema) may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may also occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. To reduce the risk of dizziness and lightheadedness, get up slowly when rising from a sitting or lying position. Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Rarely, sudden decreased vision, or even permanent blindness, may occur in one or both eyes (NAION). If this serious problem occurs, stop taking sildenafil and get medical help right away. You have a slightly greater chance of developing NAION if you have heart disease, diabetes, high cholesterol, certain other eye problems ("crowded disk"), high blood pressure, if you are over 50, or if you smoke. Rarely, a sudden decrease or loss of hearing, sometimes with ringing in the ears and dizziness, may occur. Stop taking sildenafil and get medical help right away if these effects occur. Rarely, males may have a painful or prolonged erection lasting 4 or more hours. If this occurs, stop using this drug and get medical help right away, or permanent problems could occur. Get medical help right away if you have any very serious side effects, including: increased shortness of breath or trouble breathing, fainting. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Some products that may interact with this drug are: riociguat, vericiguat. Sildenafil can cause a serious drop in your blood pressure when used with nitrates. A serious drop in blood pressure can lead to dizziness, fainting, and rarely heart attack or stroke. Do not use sildenafil with any of the following: certain drugs used to treat chest pain/angina (nitrates such as nitroglycerin, isosorbide), recreational drugs called "poppers" containing amyl or butyl nitrite. If you are also taking an alpha blocker medication (such as doxazosin, tamsulosin) to treat an enlarged prostate/BPH or high blood pressure, your blood pressure may get too low which can lead to dizziness or fainting. Your doctor may start treatment with a lower dose of sildenafil to minimize your risk of low blood pressure. Other medications can affect the removal of sildenafil from your body, which may affect how sildenafil works. Examples include azole antifungals (such as itraconazole, ketoconazole), macrolide antibiotics (such as clarithromycin, erythromycin), mifepristone, rifampin, ritonavir, among others. Do not take this medication with any other product that contains sildenafil or other similar medications for erectile dysfunction-ED or pulmonary hypertension (such as tadalafil, vardenafil).