For a bill to create strict new standards in Colorado for driving under the influence of drugs — including marijuana — the next challenge is not the contentious science behind the proposal but the price tag.

The next stop for Senate Bill 117 is the Senate Appropriations Committee, which decides whether a bill is worth its cost. In many instances, this is a perfunctory stopover on a bill’s journey through the legislature.

But SB 117 carries with it some weighty baggage: The state Public Defender’s Office estimates the bill will cost the office nearly $600,000 more per year to defend drugged-driving cases.

“The increased workload on my agency,” Douglas Wilson, the state public defender, testified before a legislative committee debating the bill last month, “will be significant, and it will be costly.”

SB 117 seeks to define what it means to be too high to drive in Colorado.

The bill would make it effectively illegal to drive with any amount of a Schedule I controlled substance — such as heroin or LSD — in the driver’s blood. It would do the same for an amount of THC — the psychoactive chemical in marijuana — above 5 nanograms per milliliter of blood. And it would create a “permissible inference” of impairment — a boost to prosecutors — in cases where people are caught with any amount of a Schedule II controlled substance in their blood. Examples of Schedule II substances include cocaine and several prescription drugs.

Opponents of the bill, especially medical-marijuana advocates, contend the science of impairment for marijuana is far from conclusive and say the bill will result in sober drivers being convicted.

“It has been noted you can have wide variation in blood levels when people have been noted not to be impaired,” said Paul Bregman, a doctor who specializes in medical-marijuana referrals.

But the bill’s supporters say most research shows the large majority of people would be impaired at 5 nanograms of THC. They say the bill is needed to stem a tide of drugged driving in Colorado.

“This is not about medical marijuana,” Thomas Raynes, the head of the Colorado District Attorneys Council, testified last month. “This is about public safety.”

Cynthia Burbach, the state’s forensic toxicologist, said the number of tests from drugged drivers coming through the state’s lab is increasing. In 2009, for instance, the state tested 8,625 samples from suspected impaired drivers, according to information Burbach provided to lawmakers. Most were tested for alcohol, but there were 791 samples that tested positive for THC, including 222 with more than 5 nanograms.

In 2011, the state tested 10,393 samples total, with 2,030 positive for THC. There were 574 with more than 5 nanograms.

The Public Defender’s Office doesn’t believe the bill will create an increased number of drugged-driving cases. Rather, it argues that the bill would require its attorneys to spend more time preparing a client’s defense and more money retesting blood samples and hiring expert witnesses.

Sen. Steve King, a Grand Junction Republican sponsoring the bill, said the Public Defender’s Office already has money in its budget for sample re-testing. He said he is working to assess the bill’s true costs.

Nonetheless, he concedes that the bill’s price tag will present a challenge in persuading lawmakers to buy into it. He expects that the bill won’t go before the Appropriations Committee until after the legislature passes the budget, which could be another month.

“There’s going to have to be some hard decisions made on how much money we have available and how we are we going to spend it,” King said.

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