There’s a number most American drivers know: 0.08. That’s the breath alcohol level that makes you legally drunk in 49 states. Fall below it, and you’re legal to drive. This number is built for a world without the edible brownie.

A new study put the combination to a controlled test. The results don’t fit the assumptions enforcement is built on – and they expose a problem that a breathalyzer cannot solve.

Researchers at the Johns Hopkins University School of Medicine (Johns Hopkins) wanted to fill a gap that has bothered the field for years.

Earlier work has examined smoked cannabis combined with alcohol, including a trial on inhaled THC and behind-the-wheel performance. But no one had run a rigorous test on cannabis edibles mixed with drinks.

Edibles act differently. They are slower to start. Their effects ramp up gradually, peak later, and linger longer than a puff from a vape pen. By the time the brownie kicks in, the drinks are wearing off.

The impairment window from an edible stretches across most of an evening. Tory R. Spindle, Ph.D., leads the Hopkins lab that designed the trial to find out just how much that overlap distorts driving.

The Hopkins team recruited 25 healthy adults who already used both substances on occasion, then brought them back for seven sessions, each a week apart.

Each session was double-blind. Volunteers ate a brownie with either no THC, a low dose, or a higher one. Then they drank enough to reach no alcohol, a moderate level, or right at the legal limit.

For context, 0.08% is the legal driving limit in 49 of 50 U.S. states. After dosing, participants ran through a driving simulator, three standardized roadside checks, and a tablet-based attention app, while blood samples tracked THC levels throughout the day. 

The simulator was the clearest mirror. Every active dose hurt driving performance except the lowest cannabis-only condition.

The largest drop appeared in the combined conditions, where impairment climbed well above what either substance produced alone. The increase was substantial.

Then came the result worth pausing on. Volunteers who ate a 25-milligram edible and drank to 0.05% – below the legal cutoff – drove worse than volunteers given enough alcohol to reach 0.08% on its own.

That comparison cuts at how American DUI law is currently written. The alcohol number does not account for the edible riding alongside it.

A driver who believes they are safe to drive because a breath test puts them under the legal limit could already be operating with more driving impairment than someone clearly over it.

The authors argue that the 0.08% threshold is likely too lenient for anyone combining the two substances.

Several other countries already use a 0.05% cutoff for alcohol alone, and Canada lowers it further when a driver has used cannabis.

Until this study, no one had directly tested whether standard roadside screens could catch a driver who was clearly impaired after using edibles. They mostly cannot, at least not at these doses.

Walk-and-turn, one-leg stand, and the eye-tracking exam together failed to register meaningful impairment in nearly every condition that involved cannabis.

They flagged the 0.08% alcohol session – the situation they were built for. They missed almost everything else.

A tablet app called DRUID, designed to catch drug-related impairment, did not perform much better. The sobriety tests simply did not track with what the simulator was recording.

Roadside blood numbers fared no better. The highest impairing dose – 25 milligrams of THC – produced blood THC levels that peaked at only 3.21 nanograms per milliliter.

That was lower than the threshold some states use to declare a driver impaired.

Regular cannabis users often walk around with similar levels while feeling nothing at all, a finding emphasized in a National Academies review of cannabis science.

Edibles also stretched out the danger. Driving deficits appeared about 90 minutes after the brownie, peaked at 3.5 hours, and lingered through 5.5 hours. The effects remained long after the drinks had cleared.

Combined with alcohol, the window widened on both ends. Blood THC and breath alcohol behaved the same whether the substances were consumed together or apart.

Whatever drove the added impairment, it wasn’t happening in the bloodstream.

Two things are clearer than they were before this trial: cannabis edibles paired with a drink or two can produce driving impairment that exceeds the legal alcohol limit, and the tools officers carry to the roadside will often fail to catch it.

That gap puts pressure on lawmakers and public-health agencies to develop better impairment detection and to rethink whether one alcohol number still makes sense in places where edibles are sold in storefronts.

The study is published in JAMA Network Open.

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