• Viagra is a brand (trade) name for the drug sildenafil.
• Viagra (sildenafil) works by blocking 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. Viagra is approved to treat erectile dysfunction (impotence) in men. Another brand of sildenafil (Revatio) is approved to treat high blood pressure associated with pulmonary arterial hypertension (PAH). Revatio is not interchangeable with Viagra.
• Viagra belongs to the class of medicines known as PDE-5 inhibitors.

• Viagra is approved to treat erectile dysfunction (ED), also known as impotence, in men. It increases blood flow to the penis following sexual stimulation and allows men to keep an erection hard enough for sex.
• Viagra only works following sexual stimulation. It works within 30 to 60 minutes, but may be taken up to four hours before sexual activity.

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.
• 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 Viagra have had bleeding events; however, it has not been established if this is actually related to Viagra use.
• Rarely, may decrease blood flow to the optic nerve causing sudden vision loss. The risk is greater in people aged older than 50, or 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).
• May interact with some medications including certain antibiotics and antifungals, herbal remedies, HIV medicines, and drugs used in the treatment of high blood pressure or prostate disorders.
• Viagra offers no protection against sexually transmitted diseases.
• Viagra is available as a generic under the name sildenafil.
• Not for use in females.

Note: 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. View complete list of side effects

• Do not take Viagra more than once daily.
• The usual recommended dosage is 50mg (range 25mg to 100mg), taken when needed, an hour before sexual activity. However, it may be taken anywhere from 30 minutes to four hours before sexual activity. Do not exceed the maximum recommended dose.
• Do not consume grapefruit or grapefruit products while taking Viagra.
• 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 Viagra if you are also taking a nitrate drug (such as nitroglycerin or isosorbide dinitrate/mononitrate) for chest pain or heart problems. Do not take Viagra 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 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 Viagra. 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 Viagra may lower blood pressure and decrease sexual performance.
• Viagra is not intended for women.

• Peak plasma concentrations of Viagra are reached within 30 to 120 minutes of a single dose. The onset of effect takes about 30 to 60 minutes alongside sexual stimulation.

Medicines that interact with Viagra may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with Viagra. 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 Viagra 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, sildenafil, 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 Viagra 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 Viagra. Grapefruit juice may also increase the side effects of Viagra.

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