The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Sildenafil and Viagra both work by inhibiting the PDE5 enzyme, which controls muscle contraction in the penis. Medications that are known to interact with sildenafil include nitrates, which are often prescribed for chest pain, and guanylate cyclase stimulators such as Adempas riociuguat for pulmonary hypertension. However, the dose is usually not more than 80 mg 3 times a day. This medicine comes with a patient information leaflet. If you prefer, you may contact the U.Products and services Erectile dysfunction: Viagra and other oral medications By Mayo Clinic Staff Medicines that you take by mouth are called oral Missing: usa. Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. Missing: usa. sildenafil citrate or viagra Savings, Coupons and Information. | SILDENAFIL (sil DEN a fil) treats erectile dysfunction (ED). It works by increasing blood flow to the penis, which Missing: usa.