Combining PDE5 inhibitors with nitrates can cause a severe drop in blood pressure, leading to dangerous side effects like dizziness, fainting, or even heart attack or stroke. Therefore, people taking any form of nitrate medications should never use PDE5 inhibitors. Sexual activity itself can put strain on the heart, so combining this with a PDE5 inhibitor may be too risky for those with these underlying heart conditions. While PDE5 inhibitors do not typically cause permanent vision loss, caution should be exercised in people with certain inherited eye diseases. Symptoms of an allergic reaction could include swelling, rashes, or even more serious issues like difficulty breathing.