Final answer:
Horace Wells introduced nitrous oxide into dentistry for pain control.
Step-by-step explanation:
The drug that Horace Wells introduced into dentistry for pain control was nitrous oxide, also known as laughing gas. Nitrous oxide is a general anesthetic that can induce a reversible loss of consciousness, making it useful for dental procedures where pain needs to be managed. It is commonly administered by inhalation and has been used in dentistry since the mid-19th century.