Make sure you have discussed any other medications you are currently taking or plan to take before starting sildenafil with your doctor and pharmacist. John’s wort. Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Protein binding is independent of total drug concentrations. Similar values for pharmacokinetic parameters were seen in normal volunteers and in the patient population, using a population pharmacokinetic approach. Take sildenafil exactly as directed.Still, if your doctor says it’s okay for you to use and you end up paying out-of-pocket, you can still get the generic for a lot less than Viagra. According to price-comparison site 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. The average length of time Viagra lasts. On average, Viagra lasts between 2 and 3 hours before its effects start to diminish. Viagra can last up to 5 hours or longer depending on your dosage, your.