Final answer:
Epinephrine is a common vasoconstrictor used in dentistry to increase the effectiveness of local anesthetics and prolong their action.
Step-by-step explanation:
The common drug used in dentistry that acts as a vasoconstrictor is epinephrine. Epinephrine is often combined with local anesthetics, such as lidocaine or novocain, to increase their effectiveness and prolong the duration of their action. It constricts the blood vessels in the area where it is injected, reducing bleeding and allowing for a longer-lasting numbing effect.