It is mandatory to check your doctor before taking this medicine to ensure that it is safe for you. It is recommended that you take your prescription exactly as directed. The doctor at your clinic will generally inject it into your body. You can also take it at home if you have been trained or are familiar with the right administration procedure. Also, keep in mind that you should take more time than is required. It is usually used three times a day for PAH, with a 4-6 hour break between each dose. You should only take them when you need them for erectile dysfunction, which is usually half an hour to an hour before any sexual activity.