Shake the liquid well for 10 seconds before each use to mix the medication evenly. Take sildenafil at around the same times every day. Take sildenafil exactly as directed. Ask your doctor if you have any questions. Amphetamines Antibiotics, especially metronidazole Drywall Counterfeit Viagra may also have too much of the active ingredient sildenafil.No, you should not take Viagra if you don’t have ED. Viagra is a prescription medication that can have serious side effects. And it’s approved only to treat ED, not to be used . 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 . Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men.