If you prefer, you may contact the U. If you have an erection that lasts more than 4 hours, get medical help right away. This medicine is available on prescription and you can also buy it from most pharmacies. This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. Patients should always ask their doctors for medical advice about adverse events. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication. · The highest recommended Viagra dose for treating erectile dysfunction (ED) is mg per day, according to the drug’s manufacturer. Doses of mg or mg would be considered off-label use. 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. We would like to show you a description here but the site won’t allow more.