Most men tolerate erectile function drugs fairly well. Some common side effects are as follows:

• Headache
• Nasal congestion, runny nose
• Back pain, muscle aches
• Flushing, redness in the face
• Indigestion
• Dizziness

The following side effects are less common, but more serious. They should receive medical attention right away:

• Vision changes, such as blurriness, blue haze, or difficulty distinguishing between certain colors
• Hearing problems
• Priapism (an erection that lasts for more than four hours)