The issue came up today at the taping of a forum at WTTW TV in Chicago.
A group of 30 high school students grilled Obama on issues ranging from drug use to abortion to education funding. Obama has acknowledged that he smoked marijuana in high school and college and used cocaine "a few times."
Wednesday, the Democrat told students that he stopped after professors he respected questioned him about what he planned to do with his life. He says that led him to focus more on his community and distracted him from drug use.
Republican Senate candidate Alan Keyes is scheduled to speak at the same forum Wednesday afternoon.