Colorado lawmakers have rejected an initial effort to cap the potency of marijuana that customers can buy at recreational pot stores.

Rep. Kathleen Conti, R-Littleton, had proposed barring stores from selling marijuana and marijuana products — including concentrates — that contain more than 15 percent THC. That amount is below the average potency of products currently sold in recreational stores.

Late Wednesday, lawmakers on the House Finance Committee narrowly voted down the proposal, on a 6-5 vote. But that decision may not be the end of the debate — for this year or for next.

The proposed limit was pitched in an amendment to a bill — House Bill 1261 — reauthorizing Colorado’s rules for recreational marijuana stores. Wednesday’s hearing was the bill’s first, meaning there will be multiple other opportunities this legislative session to try again to insert the amendment.

And even those who voted against the amendment on Wednesday expressed support for some type of potency cap, after more study.

“We’ll be revisiting this next year, for sure,” Rep. KC Becker, a Boulder Democrat who was a no vote on the amendment, said at Wednesday’s hearing.

The available potency of marijuana products has boomed in Colorado’s commercial market. A study by the state last year found that the average potency of raw marijuana sold in Colorado stores is 17.1 percent THC. The average potency of concentrated marijuana — a form increasingly popular with experienced consumers — is 62.1 percent THC.

Those numbers concern lawmakers and others who worry about the impacts of such potencies, especially on teens who may be illegally acquiring the products.

“We do not know how this affects the brain, especially the developing brain of our kids,” Conti said Wednesday. “… I think we need to proceed with caution.”

Marijuana industry supporters, though, testified during Wednesday’s four-hour hearing that capping potency could drive consumers to the black market or could lead to people trying to make volatile concentrates at home, creating the risk of explosions. Another possibility, they said, is that consumers would simply consume more of the lower-potency marijuana.

Lawmakers also raised questions about how state officials would police the potency limits.

“I just don’t see how this would work,” said Rep. Lois Court, D-Denver.

After rejecting the potency cap, lawmakers on the Finance Committee unanimously passed the rules bill.

In addition to reauthorizing the rules for recreational marijuana stores, the current version of the bill also gets rid of an added limitation on out-of-state customers. Right now, people without Colorado identification can only buy up to a quarter-ounce of marijuana at a time. If the bill passes as it now stands, that would change and out-of-state customers could buy up to one ounce, the same as customers from Colorado.

The bill is scheduled to be heard next in the House Appropriations Committee.

Meanwhile, a proposed ballot initiative that would also cap potency of marijuana products at recreational stores is still going through the approval process to be placed before voters.

John Ingold: 303-954-1068, jingold@denverpost.com or @johningold