“There should be no controversy about the existence of marijuana addiction,” said David Smith, a physician who has been treating addiction since he opened a free clinic in San Francisco’s drug-drenched Haight-Ashbury neighborhood in the 1960s. “We see it every day. The controversy should be why it appears to be affecting more people.”

Although estimates of the number of people who use marijuana vary, the federal government and the marijuana industry tend to agree that total marijuana use has remained relatively constant over the past decade. Increased use in the past three years has been slight, despite increased commercial availability in states that have legalized it.

The percentage of people who become addicted to marijuana — estimated at about 9 percent of all users, and about 17 percent of those who start in adolescence — also has been stable. Some studies report that even higher proportions of marijuana users develop a dependence, which means they experience withdrawal symptoms when they stop using the drug.