This medicine also works best when you take it an hour before sex on an empty stomach. It’s available in 25 mg, 50 mg, and mg tablets and lasts up to 18 hours. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. You can take it in a small dose daily or in a larger dose as needed. This medicine works best when you take it on an empty stomach an hour before sex. Common ones include: Flushing of the skin. Do not take a double dose to make up for a missed one.The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 . Medications Prescribed: Viagra, Cialis, sildenafil, tadalafil Generic offered: Yes Why We Chose It. 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 .