DENVER — Colorado gave a lonely reception to marijuana when it became the second state to legalize the drug. Just as state leaders planned.

On Monday, Governor John Hickenlooper quietly removed the final barrier to legalization by declaring that an amendment passed by voters in November was officially part of the state constitution. He announced the move on Twitter and e-mail after the fact. In response, a handful of marijuana activists celebrated by toking up on the Capitol steps, but there were no crowds and little fanfare.