If you prefer, you may contact the U. Do not take a double dose to make up for a missed one. Make sure you have discussed any other medications you are currently taking or plan to take before starting sildenafil with your doctor and pharmacist. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Concomitant administration of strong CYP3A4 inducers, such as rifampin, is expected to cause greater decreases in plasma levels of sildenafil. A stronger CYP3A4 inhibitor such as ketoconazole or itraconazole could be expected to have greater effect than that seen with saquinavir.Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision. The recommended dosage of Viagra for erectile dysfunction (ED) is 50 mg, taken as needed about 1 hour before sexual activity. You can take it 30 minutes to 4 hours before sexual activity. And you.  · But Viagra may be taken between 30 minutes and 4 hours before you anticipate sexual activity. Viagra tablets may be taken with or without food. Keep in mind that meals high in fat may cause Viagra.