The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Trimazosin Other Interactions Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Age greater than 50 years or Coronary artery disease or Crowded disc or low cup to disc ratio in the eye an eye disorder or Diabetes or Hyperlipidemia high fats in the blood or Hypertension high blood pressure or Non-arteritic anterior ischemic optic neuropathy or NAION serious eye condition , history of or Smoking—May increase the chance for a serious side effect in the eye called NAION. Do not take more or less of it or take it more often than prescribed by your doctor. Place the tip of the syringe into you or your child’s mouth and point it towards the cheek. Do not use this medicine if you are also using a nitrate medicine, often used to treat angina or high blood pressure.Dosage for erectile dysfunction 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. Side Effects. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may.  · a light-headed feeling, like you might pass out. Common sildenafil side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; heartburn, nausea, or upset stomach. abnormal vision (blurred vision, changes in color vision) runny or stuffy nose, nosebleeds; sleep problems (insomnia); or.