Both sildenafil and the metabolite have terminal half lives of about 4 hours. It is important to tell your doctor about any heart problems you have now or may have had in the past. Population pharmacokinetic data from patients in clinical trials also indicated a reduction in sildenafil clearance when it was co-administered with CYP3A4 inhibitors such as ketoconazole, erythromycin, or cimetidine [see Dosage and Administration 2. Distribution and use of this material are governed by our Subscriber Agreement and by copyright law. The mean steady state volume of distribution Vss for sildenafil is L, indicating distribution into the tissues. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food.Answer 1 of 5: Can anyone tell me whether Viagra or Kamagra are available in HCMC or Hanoi over the counter in a pharmacy. Or is a prescription required? Vietnam. 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 test1.ru Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash.