Have an accurate diagnosis and prescription: Remember that Viagra and generic sildenafil are not available over the counter — you will need a prescription. Some .The mechanism of action for ED pills is simple: They inhibit the PDE5 enzyme which regulates the signals responsible for the smooth muscle tissue. Do not take more or less of it or take it more often than prescribed by your doctor. Sildenafil comes as a tablet and suspension liquid to take by mouth. You can see an in-person provider or speak with a doctor virtually. John’s wort. Do not drink grapefruit juice if you’re taking sildenafil Revatio for pulmonary hypertension.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.  · Take sildenafil exactly as it was prescribed for you. Follow all directions on your prescription label and read all medication guides or instruction sheets. Revatio is usually taken three times each day, about 4 to 6 hours apart. Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. AdGet a Prescription for ED treatment, discreetly deliverED to your door.Erectile dysfunction can be caused by a variety of factors , including age, cardiovascular problems, diabetes, and other conditions that affect nerve function or blood flow. Shake the liquid well for 10 seconds before each use to mix the medication evenly. Flibanserin is not approved in Australia. What should I do if I forget a dose? Make sure you have discussed any other medications you are currently taking or plan to take before starting sildenafil with your doctor and pharmacist. If you’re not sure if you’re taking a nitrate, ask your doctor or pharmacist. Take sildenafil as needed before sexual activity.