The body takes longer to absorb it after a high-fat meal or alcohol. Fear of intimacy and anxiety about sexual performance can affect your ability to get and maintain an erection. This medication is taken with or without food an hour before sex. It works for up to 36 hours. It is completely safe to take Viagra or Sildenafil provided you are male and over, and have been prescribed it for erectile dysfunction.Make sure you get the right prescription and dose. When you order medicines and when you receive them in the mail, make sure they’re the exact dose and type prescribed by . You should not use Viagra if you are allergic to sildenafil, or: if you take other medicines to treat pulmonary arterial hypertension, such as riociguat (Adempas). if you take nitrates. . Viagra treats erectile dysfunction (ED) in adults. It’s usually taken about 1 hour before sexual activity. Below, you’ll find information about recommended dosages and how to .