A bill to set a threshold for driving under the influence of marijuana advanced in the state Senate yesterday (Mon 4.18.11). But Colorado Public Radio Health Reporter Eric Whitney says senators want more study before deciding what exactly the limit should be.

WHITNEY: When the House passed the marijuana DUI bill, it said anyone with a level of five nanograms of THC per milileter of blood is too high to drive. THC is the psychoactive component of marijuana.

Sarah Urfer was one of two toxicologists who told the Senate committee Monday that five nanograms is a reasonable number.

URFER: At five we are still strongly supported by the science, and allowing probably more than is even necessarily required leeway for the medical marijuana community.

WHITNEY: Medical marijuana advocates worry that if the THC level is set too low, people who use the drug regularly will be unfairly arrested. Dr. Paul Bregman, a medical marijuana specialist, says some regular users can probably drive safely with much higher levels.

BREGMAN: It needs more research, but I would think in those terms, at least above 20.

WHITNEY: Advocates for and against the five nanogram limit gave the committee copies of scientific studies they say supported their positions. Some members found the conflicting studies confusing. Chairwoman Morgan Carroll.

CARROLL: I'll be the first to say, I'm not a scientist. I mean, I'm looking at competing research, and if we're about to pick a number, I feel a little arbitrary.

WHITNEY: Police and prosecutors endorsed the five nanogram limit, but in the end the committee amended the bill to ask a state task force to evaluate the science for them, and then recommend  appropriate THC blood level. The amended bill passed unanimously and now goes to one more committee before the Senate as a whole takes it up.