DENVER (AP) — Marijuana for recreational use became legal in Colorado on Monday when the governor took the procedural step of declaring the voter-approved change part of the state constitution.

Colorado became the second U.S. state after Washington to allow pot use without a doctor’s recommendation. Both states prohibit public use of the drug, and commercial sales in Colorado and Washington won’t be permitted until after regulations are written next year.

Hickenlooper, a Democrat, opposed the measure but had no veto power over the voter-approved amendment to the state constitution. He tweeted his declaration Monday and sent an executive order to reporters by email after the fact, preventing a public countdown to legalization as seen in Washington.

“Voters were loud and clear on Election Day,” Hickenlooper said in his statement. The law allowed him until Jan. 5 to declare marijuana legal.

Adults over 21 in Colorado may now possess up to an ounce (28.35 grams) of marijuana, or six plants. Public use and sale of the drug remain illegal.

Colorado and Washington officials both have asked the U.S. Department of Justice for guidance on the laws that conflict with federal drug law. So far the federal government has offered little guidance beyond stating that marijuana remains illegal and that the controlled Substances Act will be enforced. Of special concern for state regulators is how to protect state employees who violate federal drug law by complying with state marijuana laws.

Colorado’s marijuana measure, Amendment 64, was approved with 55 percent of the vote last month. One of the authors of Colorado’s pot amendment, Mason Tvert, called the declaration “truly historic.”

“We are certain that this will be a successful endeavor and Colorado will become a model for other states to follow,” Tvert said in a statement.

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Copyright 2012 The Associated Press.