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Sildenafil: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Feb 25, 2019.

1. How it works

  • Sildenafil blocks the action of an enzyme known as cGMP-specific phosphodiesterase type-5 (PDE-5). This enzyme breaks down cGMP, a substance that induces smooth muscles to relax. Blocking PDE-5 means increased levels of cGMP within the smooth muscles which promotes muscle relaxation and vasodilation (a widening of blood vessels).
  • High levels of PDE-5 are found in the penis, lungs, and retina. PDE-5 is also found throughout the body within the smooth muscle cells of blood vessels and muscles.
  • Sildenafil belongs to the class of medicines known as PDE-5 inhibitors.
  • Viagra, a brand of sildenafil, is approved to treat erectile dysfunction (impotence) in men.
  • Revatio, another brand of sildenafil, is approved to treat pulmonary arterial hypertension and improve exercise capacity in both men and women.

2. Upsides

  • Sildenafil (Viagra brand) increases blood flow to the penis following sexual stimulation. It does this by blocking the enzyme responsible for the breakdown of cGMP. cGMP is a substance that relaxes and widens blood vessels and smooth muscle cells. This allows men to keep an erection hard enough for sex.
  • Sildenafil (Viagra brand) only works following sexual stimulation. It works within 30 to 60 minutes, but may be taken up to four hours before sexual activity.
  • Sildenafil (Revatio brand) reduces high blood pressure associated with pulmonary arterial hypertension (PAH). This improves the ability to exercise in people with PAH and delays any worsening of their symptoms.
  • Revatio is available as a generic under the name of sildenafil (20mg tablet). There is also a generic version of Viagra available, under the name sildenafil, in 25mg, 50mg, and 100mg strengths.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Dyspepsia, flushing, headache, indigestion, muscle or back pain and visual disturbances are commonly reported.
  • Other side effects include a drop in blood pressure, insomnia, nasal congestion or a runny nose and nosebleeds. Sudden hearing loss has also been reported. Some people taking sildenafil have had bleeding events; however, it has not been established if this is actually related to sildenafil use.
  • Rarely, may decrease blood flow to the optic nerve causing sudden vision loss. The risk is greater in people aged older than 50, with diabetes, heart disease, high blood pressure, high cholesterol, who smoke, or with certain pre-existing eye problems.
  • Rarely, may cause other side effects or lead to painful erections lasting longer than four hours. See your doctor immediately if this happens.
  • May not be suitable for some people including those with angina, chest pain, or other heart conditions, liver or kidney problems, blood or bleeding disorders, stomach ulcers, certain eye conditions, or a physical deformity of the penis (such as Peyronie's disease). Higher dosages of sildenafil are associated with a higher risk of death in children with PAH.
  • May interact with some medications including certain antibiotics and antifungals, herbal remedies, HIV medicines, nitrates, and drugs used in the treatment of high blood pressure or prostate disorders. Sildenafil should not be taken with other PDE-5 inhibitors.
  • Sildenafil offers no protection against sexually transmitted diseases.

Notes: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. For a complete list of all side effects, click here.

4. Bottom Line

Sildenafil is used to treat erectile dysfunction (ED) and to reduce blood pressure in pulmonary arterial hypertension (PAH). It is taken when needed for ED and on a daily basis for PAH. A headache, stomach upsets, and flushing are the main side effects.

5. Tips

  • If you are taking sildenafil (Viagra) for ED, do not take more than once a day. If you are taking sildenafil (Revatio) for PAH, take as directed (usual dosage is three times daily).
  • Do not consume grapefruit or grapefruit products while taking sildenafil.
  • Always talk to your doctor before taking ANY medication for ED. Some ED medications are not suitable for men with certain medical conditions, and your doctor can advise you on an alternative.
  • Call your doctor immediately if you experience dizziness or feel nauseous during sexual activity, or develop pain, numbness, tingling in your chest, arms, neck, or jaw.
  • Do not take sildenafil if you are also taking a nitrate drug (such as nitroglycerin or isosorbide dinitrate/mononitrate) for chest pain or heart problems. Do not take sildenafil with recreational drugs such as "poppers". Taking these substances together can cause a sudden and dangerous drop in blood pressure.
  • Always buy your ED medications from a reputable supplier as there are a lot of counterfeit products on the market.
  • Certain lifestyle and psychological factors, such as smoking, drinking, and stress, can impact on ED. Eliminating these - for example, staying more active, losing weight if you are overweight, reducing alcohol intake, stopping smoking and reducing stress - may also help improve ED.
  • Never take other ED medications at the same time as taking sildenafil. Do not take Viagra while taking Revatio and vice-versa.
  • Seek urgent medical advice if an allergic reaction, sudden vision loss or painful erections lasting more than four hours occur.
  • Drinking alcohol after taking sildenafil may lower blood pressure and decrease sexual performance.

6. Response and Effectiveness

  • Peak plasma concentrations of sildenafil are reached within 30 to 120 minutes of a single dose. Effects of sildenafil, when used for ED, are usually noticeable within 30 to 60 minutes of a single dose.
  • When used to treat PAH, blood pressure lowering effects are noticeable within one to two hours.

7. Interactions

Medicines that interact with sildenafil may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with sildenafil. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with sildenafil include:

  • alpha-agonists (eg, clonidine, methyldopa)
  • alpha-blockers (eg, alfuzosin, doxazosin, prazosin, tamsulosin, or terazosin)
  • antibiotics, such as clarithromycin, erythromycin, metronidazole, norfloxacin, rifabutin, rifampin, or tetracycline
  • antidepressants, such as desipramine, sertraline, or St. John's Wort
  • antifungal medications, such as itraconazole, ketoconazole, or voriconazole
  • blood thinners, such as warfarin
  • cystic fibrosis medications, such as ivacaftor
  • epilepsy medications, such as carbamazepine, oxcarbazepine, phenobarbital, phenytoin, or primidone
  • other erectile dysfunction medications (eg, alprostadil, Cialis, vardenafil)
  • heart medications, such as amlodipine, amiodarone, atenolol, bosentan, candesartan, captopril, clonidine, diltiazem, enalapril, irbesartan, losartan, nifedipine, propranolol, sotalol, or verapamil
  • hepatitis medications, such as boceprevir and simeprevir
  • HIV medications (eg, atazanavir, delavirdine, efavirenz, fosamprenavir, indinavir, etravirine, ritonavir, nevirapine, saquinavir, or tipranavir)
  • nitrates (eg, amyl nitrate, isosorbide dinitrate, isosorbide mononitrate, or nitroglycerin)
  • others, such as aprepitant, butalbital, cimetidine, cyclosporine, or imatinib.

In addition, taking sildenafil with too much alcohol can lead to orthostatic hypotension, which is a drop in blood pressure when you go from a sitting or lying down position to standing. Symptoms include a headache, dizziness, or fainting when standing. Antacids, such as magnesium hydroxide or aluminum hydroxide may also affect the absorption of sildenafil. Grapefruit juice may also increase the side effects of sildenafil.

Note that this list is not all-inclusive and includes only common medications that may interact with sildenafil. You should refer to the prescribing information for sildenafil for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use sildenafil only for the indication prescribed.

Copyright 1996-2020 Revision date: January 26, 2020.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.