Viagra (Sildenafil) is a prescription medication used to treat erectile dysfunction (ED) in men by increasing blood flow to the penis, helping to achieve and maintain an erection during sexual stimulation. Available in 25mg, 50mg, and 100mg blue diamond-shaped tablets, it is a PDE5 inhibitor typically taken 30-60 minutes before sexual activity. 

Cialis (Tadalafil) is a prescription medication primarily used to treat erectile dysfunction (ED) and symptoms of benign prostatic hyperplasia (BPH) by relaxing blood vessels and muscles in the prostate and bladder. Known for its long duration, it can last up to 36 hours. It is available in dosages ranging from 2.5 mg to 20 mg. 

Vardenafil (formerly brand name Levitra) is a prescription oral PDE5 inhibitor used to treat erectile dysfunction by increasing blood flow to the penis during sexual stimulation. Available in 2.5mg to 20mg tablets (including orally disintegrating forms), it is taken 25-60 minutes before activity, with effects lasting up to 5 hours.