Smartphone sensors can be used to detect if you are high on marijuana with 90 percent accuracy

Determining when someone is too high to drive is a problem that has evaded the chemists, psychiatrists, law enforcement agents and public policy makers.

Scientists explained to Daily Mail Online why the most scientific tests may not be the best way to determine how high is too high and the unique chemistry that makes marijuana a drug-screening enigma.

Some are unique to the drug, but one is true of just about any substance: tolerance.

Depending on how often and in what doses someone uses any substance, the amount it takes to for the drink or drug to have an effect varies drastically.

So, people with medical marijuana prescriptions, for example, who might smoke weed every day, seem totally unaffected and breezily pass a field sobriety test – which involves tests for coordination and balance, like walking the line, as well as some for memory and attention – even though they’ve recently ingested lots of the drug.

Beyond that, marijuana moves through the body in a very different way from alcohol.

Marijuana – more specifically, its psychoactive component – leaves the blood very quickly, but it lingers in the fat and brain, meaning its cognitive effects do, too.

According to Dr Richard Clark, director of the division of medical toxicology at the University of California, San Diego, marijuana may even move from these tissues back into the blood days later in ‘chronic’ smokers.

And just to add an extra level of difficulty, the THC in increasingly popular edibles gets converted quickly to anther compound in the stomach, so a THC test might not even detect it, even when a high was in full effect.

Blood and urine tests are available, but sometimes a long time passes between when someone is pulled over and when the test can be administered.

There are two recently developed breathalyzers for THC – one from Hound Labs and another from Cannabix – and several other tests in development, but these face the same challenges of disparity between blood level and actual high.

‘Field sobriety testing introduces subjectivity into something you’d really like to be subjective,’ says Dr Hall.

For now, however, no method is perfect. 

‘So what does it all mean,’ Hall said, ‘except it’s better to drive completely sober.’

-Natalie Rahhal, Deputy Health Editor for

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