It works for up to 36 hours. Since alcohol decreases blood flow to the penis, it limits the drug’s effectiveness i. Fact Checker: Daniel Boyer M. Common ones include: Flushing of the skin. Usually, it takes between 30 and an hour for Viagra to start working, and lasts for four to six hours. · 2. Rex MD – Free Viagra Samples. Rex MD is a men’s health-focused telemedicine service. They provide free prescription pharmaceutical samples for .  · Drugs including Viagra and Cialis lead to an average improvement of 4 to 8 points on the scale used by researchers and clinicians. The team behind the new study . Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body .