Typically this takes the form of a tick box medical questionnaire, which is then reviewed by the pharmacist prior to supply. Common ones include: Flushing of the skin. Side Effects Are Minimal. You can take this medicine with or without food 30 minutes before sex, depending on the dose. This applies, for example, when you want to take Tadalafil brand name Cialis instead of Viagra. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food.Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a . Oral medications, like Viagra and Cialis, which require a prescription and are commonly used to treat ED, can take 30 minutes to take effect, she adds. Injections . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men.