Answer:
Beta-blockers are medications prescribed by physicians or mostly cardiologists who cater to individuals with heart-related issues. These issues include things that limit the ability of the heart to function properly and they include people suffering from heart attacks, high blood pressure and other illness that requires proper functioning of the heart.
However, Beta-blockers when administered slows the heartbeat per minute thereby reducing the amount of oxygen needed, but due to the way exercise raises the beat of the heart, oxygen pumped to the heart, it requires a well functioning heart to be able to meet up with the demand of the heart required for exercise. So, Beta-blockers already active in the bloodstream would affect cardiac output exposing the heart to further slow down this could result in dizziness has the heart couldn't handle the pressure required to pump more for an exercise.