DENVER—Before Colorado became the first state to allow marijuana for recreational purposes, supporters boasted that legalization would generate a sizable tax windfall, while opponents warned that it could have dramatic social consequences.

Slightly more than a year into the state’s experiment with sanctioning pot sales to adults 21 and older, neither prediction is proving entirely true. Marijuana so far hasn’t been the boon or bane that many expected, offering potential lessons to other states considering legalization.

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