What are illegal beta blocker?

Ergogenic aids - illegal: beta-blockers Beta-blockers are used clinically to control migraines, cardiac arrhythmia and decrease blood pressure. They prevent the effect of beta - receptors (nerves found in the heart and blood vessels). Beta-blockers slow the heart rate, and reduce tension. They are unlikely to be used in active sports, however, they have been used in shooting, archery etc. where accuracy is vital. This works by reducing tremors and also reducing the heart rate significantly so that shots may be fired, or arrows shot between each heart beat. Dangers of misuse and side effects include: athlete losing their natural control of heart rate lower performance during lengthy events

What are beta blockers?