Take sildenafil exactly as directed. If you are taking sildenafil for PAH, take the missed dose as soon as you remember it. You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox. Other uses for this medicine This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. If you ever need emergency medical treatment for a heart problem, the healthcare providers who treat you will need to know when you last took sildenafil.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. Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum recommended dose is mg. Don’t take sildenafil (Viagra) more than once daily (at any dose). Written by Brian Leonard, PharmD, BCACP, BCGP. · You can take the drug 30 minutes to 4 hours before you anticipate sexual activity. Based on how well you respond to this dosage and any side effects you have, your doctor may either increase your.