A brownie, a drink, a drive: Hidden impairment standard sobriety tests completely miss

A Johns Hopkins-led study published in JAMA Network Open on May 1 found that cannabis edibles combined with alcohol can impair driving more than either substance alone, while standard field sobriety tests can miss some of the impairment.

The within-participant crossover trial examined cannabis edibles, alcohol and the two together. According to Johns Hopkins and the journal, the combination produced greater driving impairment and stronger subjective intoxication than either substance on its own.

The research also found that standard field sobriety tests did not reliably detect impairment in cannabis-alone conditions or in some lower-alcohol co-use conditions. That finding raises concerns about roadside detection, especially when someone has consumed both alcohol and an edible before driving.

Johns Hopkins said the results support better methods for detecting impairment and more public education about the risks of mixing cannabis and alcohol. The study adds to a growing public-safety debate over how police, doctors and lawmakers can identify drivers who are impaired even when routine tests do not clearly show it.

The authors’ findings may matter most in situations where alcohol use is near legal limits. Johns Hopkins said the co-use effect was stronger than alcohol alone at the 0.08% legal limit in the U.S.

Why it matters

The study suggests that relying on standard roadside sobriety checks alone may leave some impaired drivers undetected when cannabis edibles are involved. That could affect enforcement, public health messaging and future research on drug-alcohol impairment.