MONTPELIER, Vt. (AP) - The Vermont Senate gave preliminary approval Wednesday to a bill to legalize the possession of up to an ounce of marijuana by adults in the state.

Approval came on a 16-13 roll call vote after about two hours of debate.

Supporters said that marijuana prohibition has failed and that a system of regulating and taxing the drug would provide more control. They said teenagers report pot currently is easier to get than alcohol.

Opponents said Vermont should not be legalizing another drug at a time when the state, like many others, is fighting a surge in opiate abuse.

Senate President Pro Tem John Campbell, a Windsor County Democrat, argued that opiate abuse had reached “epidemic proportions” in Vermont.

“Shouldn’t we be trying to solve that problem first, before we introduce another drug that we all have to admit has mind-altering characteristics?” he asked.

Others argued that the public is already well acquainted with marijuana, citing a recent Rand Corp. study that found that more than 80,000 of the state’s residents had used marijuana within the previous month of the survey.

“With over 80,000 Vermonters admitting to using marijuana on a monthly basis, it could not be more clear that the current system is broken,” Democratic Gov. Peter Shumlin, who supports legalization, said after the Senate vote.

The Senate was expected to vote on final passage of the bill Thursday.

The measure would allow possession of up to an ounce, with licensed growers supplying retail marijuana outlets. The measure includes a 25 percent excise tax on marijuana. It does not allow marijuana-laced candies or other edibles. It also does not allow homegrown pot.

The no-homegrown provision drew criticism from some senators, and prompted at least one to vote against it. Sen. Rebecca Balint, D-Windham, said she supported legalization in principle.

“But I stand here today, reluctantly, to say that I will be voting against this bill … because I believe this bill does not allow room for the homegrown folks in my county and the smaller growers who would like to be part of this new, viable business,” she said.

If the House approves the bill and Shumlin signs it, Vermont would become the first state to legalize marijuana by state law, while the federal government still outlaws it. Marijuana has been legalized by referendum in Colorado, Washington state, Oregon and Alaska. At least three states - California, Maine and Massachusetts - are expected to vote on legalization referenda this year.

The measure’s prospects in the House are uncertain. Shumlin’s fellow Democrats control both House and Senate, but his clout is believed to have diminished somewhat since he announced in June that he would not seek a fourth two-year term this year.

House Majority Leader Sarah Copeland-Hanzas, D-Bradford, said the bill faces uncertain prospects in the Judiciary and the Appropriations committees. Lawmakers have a weeklong break next week surrounding the state holiday of Town Meeting Day.

“I’m asking people to go home and engage with their neighbors about what is legalization, what is the proposal that’s on the table,” Copeland-Hanzas said.