A prototype app called “Am I Stoned?” aims to answer that very question. This could be helpful as legal marijuana becomes more accepted and widespread in the U.S. Knowing how impaired they really are may help marijuana users make safer choices about activities that may be difficult or dangerous while high.

The researchers involved were given government funding to develop the app prototype as a potential field sobriety test. The research team leader is Harriet de Wit, a professor in the Department of Psychiatry and Behavioral Neuroscience at The University of Chicago. The scientists presented their findings at the annual Experimental Biology conference. The results have not yet been published in a peer-reviewed journal.

The aim of the prototype is to make users more aware of the effects of THC (tetrahydrocannabinol), the compound in marijuana responsible for its psychoactive effects. THC interacts with the brain, binding to cannabinoid receptors in regions of the brain associated with coordination, memory, cognition, and time perception. THC has been found to temporarily inhibit performance, but the amount of the inhibition depends on the individual and the amount and concentration of the THC consumed.

The scientists recruited 24 people who regularly used cannabis, but not on a daily basis, for the study. The participants were asked to perform tasks evaluating their memory, cognitive speed, reaction time and fine-motor skills on iPhones and desktop computers a few hours after consuming a pill containing 0, 7.5, or 15 milligrams of THC. The researchers found that the desktop apps could detect impairment successfully using three of the four tasks, while iPhone apps could do so with only one of the tasks.

The next steps include fine-tuning the tasks in the app to make them even more sensitive to detecting THC impairment. Project researcher Elisa Pabon, a doctoral candidate at the Pritzker School of Medicine at The University of Chicago, said, “The effects of THC on performance may be subtle, so we need highly sensitive tasks to detect impairments.”

At this stage, there is much more data that still needs to be gathered before the app can reliably test people for THC impairment. Scientists are currently evaluating an optimized version of the app with tasks that are longer and more complex in a second-phase study now underway. They hope they can find a sweet spot in the length and complexity of the tests.