What if I get side effects on the Sildenafil 50mg dose? Do not use this medicine if you are also using a nitrate medicine, often used to treat angina or high blood pressure. Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse. When used at the same time, these medications can cause a sudden decrease in blood pressure levels that may, in some cases, be life-threatening. What Sildenafil dosages are there? Viagra can be taken with or without food. Viagra is a safe and effective medication for most men with ED.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.  · This is the normal dose of Viagra that produces improvements for most men with ED. Depending on your needs, your dosage may be adjusted to 25 or mg. If you’re prone to side effects from Viagra, your healthcare provider may lower your dosage. If you have severe ED, it might be increased to mg. Sildenafil, the active ingredient in Viagra®, is an FDA-approved medication for treating erectile dysfunction (ED). If you’re Googling dosages, you may have some misunderstandings about what it’ll do for you — and that’s okay. 21 percent at a dosage of 50mg and 28 percent at a dosage of mg in clinical trials. Other side effects.