Its effects often last for 4 to 5 hours. What should I do if I forget a dose? Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine. Sildenafil usually should not be taken more than once every 24 hours. That means eating healthy foods, exercising, getting good sleep, limiting stress, and treating other health conditions that might cause ED, like high blood pressure, heart disease, diabetes, obesity, and low testosterone, Dr.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 . 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 . In most males,* Viagra works within 1 hour after it’s taken. It’s possible that Viagra can begin working within 30 minutes of being taken. But other times, it may take up to 4 .