Do the effects of cannabis mess with cognitive ability for days after use and are urine tests the best way to sniff-out at risk employees?

Kirk Hardy has in his boardroom a bulk bag of dried parsley flakes, which is well past its use-by date.

It’s a prop, not an elderly garnish. The former police drug squad detective boasts he can estimate by sight what 14 grams – half an ounce – of parsley is.

The 14 grams is what’s proposed as the amount of cannabis a person can legally buy a day should cannabis be legalised. Hardy transfers a portion of parsley from the big bag into a smaller, snap-lock bag. There’s a fair bit of adding, eyeballing, bag shaking, then subtracting. He almost hits his goal.

His bag of parsley weighs in at 13 grams. He doesn’t say anything, but you can sense disappointment.

Hardy is the CEO and founder of The Drug Detection Agency, which specialises in workplace drug testing. If cannabis is legalised and people across the country can legally buy a 14 gram bag of cannabis every day, he expects his business will boom. 

Despite the prospect of profits he’s not personally supportive of legalisation. During his years in policing he saw cannabis linked with mental health issues. He’s also affiliated with Smart Approaches To Marijuana NZ (SAM-NZ,) an alliance of people opposing legalisation which is running the Say Nope to Dope campaign. During his years in policing he saw cannabis linked with mental health issues.

Since starting the company 15 years ago, the client base has grown to around 4500 across New Zealand and Australia. Each year the business processes around 250,000 drug tests. It offers a range of tools to businesses to help manage risks from a number of substances. These range from the usual pee-in-a-pottle or saliva tests and hair tests, to drug ‘CSI training’ courses so managers can learn how to spot telltale signs of drug use – and yes, there’s even an app to make it easy.

Roughly 5.5 percent of people the company tests have results showing they could be at risk of being impaired in some shape or form.

It’s deadly serious stuff. New Zealand has had high-profile fatal accidents linked to cannabis use.

In 1990 a bungy jump at Rainbow’s End went horribly wrong after the jump master and the jumper smoked a joint shortly before a jump. The jump-master forgot to attach the bungy cords to the jumper’s feet. The jumper died in hospital and the jump-master was found guilty of manslaughter.

In 2012 a hot air balloon collided with power lines. All 11 people on the flight died. Post-mortem toxicology of the pilot showed evidence of long-term and recent use of cannabis.

The Transport Accident Investigation Commission said it couldn’t say for sure how much cannabis-use had contributed to the pilot making an error in judgment in deciding to attempt to climb above the power lines instead of rapid descent, but said it was highly likely the pilot smoked cannabis on the morning of the flight.

What tests show

Cannabis legalisation raises a difficult issue. Toxicology tests can show past use, but pinpointing impairment through toxicology is tricky.

When inhaled, cannabis can shows up in blood soon after use, but then levels drop.

As it moves out of the system metabolites can be picked up in urine and a person who smoked a joint several days ago can return a positive urine test for cannabis. For occasional users this time period is generally short, for regular or heavy uses this can be close to 30 days.

The gap between past use and a positive result narrows for saliva tests, but saliva tests aren’t always accurate.

Because of the inability of urine tests to be linked to being stoned at the time of the test, some people have questioned whether they have a place in workplace testing for cannabis if it’s legalised. There’s a risk they’ll pick up people who smoked several days ago, not people who are high at work.

Hardy argues the window between use and the risk of possible impairment is longer than people think. 

There’s a distinction in his argument between ‘intoxication’ and ‘impairment’. The initial high from consuming cannabis can last just a couple of hours after smoking weed, and a bit longer after consuming it in edible form is what he calls intoxication.

Impairment, essentially a ‘cannabis hangover’, according to Hardy can linger long beyond those few hours, affecting workers long after the initial high has worn off.

“Someone who’s using cannabis regularly, and then they stop, can still be impaired some two weeks later.”

Kirk Hardy with 13 grams of parsley flakes. Photo: Farah Hancock

A mixed bag of results

The hangover he’s referring to is cognitive impairment. Results from scientific studies into the topic are mixed, according to one review paper which looked at what studies have previously been completed.

For attention and concentration, two studies found no difference between a control group, former heavy users and current users when tested on day zero, one, seven and 24 of cannabis abstinence. Three studies showed opposite results; users who had gone 24 hours without cannabis had impaired attention and longer reaction times than people who don’t use cannabis.

For assessment of decision making and risk taking there’s only one study and it looked only at chronic, heavy users after 12 hours of last use. Compared with non-users, they took more risks and their decision making was poor. “More research is needed” was the verdict on that topic.

Four studies found users made more errors related to inhibition and persevering than non-users made after 12 to 19 hours of abstinence. Three studies, with small sample sizes, found no effect.

No studies uncovered a difference in memory between users and non-users, and there are conflicting results for verbal fluency.

What the results do show is when there are effects, they’re greater for heavier users.

United States Johns Hopkins Medicine’s professor Ryan Vandrey’s research focuses on the behavioural pharmacology of cannabis. He’s not sold on the idea of a cannabis hangover and calls current research a “mixed bag at best”.

It’s not just the range of results. Part of the problem is the way the research has been done.

Ideally you would look for a group of people who don’t use cannabis and establish a baseline before you introduce cannabis.

“But ethically you cannot take a random group of people and get half of them addicted to cannabis and then test their cognitive abilities.”

The people recruited to the “mixed bag” of studies are already cannabis users.

“There may be functional differences in the types of individuals who become daily cannabis users and their cognitive performance to begin with. We don’t know that.”

There are nuances to consider when looking at the results. Cannabis withdrawal can cause insomnia, he says. If a user’s cognitive abilities are tested after abstaining from smoking for a couple of days, a lack of cognitive function could be related to not sleeping properly, rather than a residual effect of cannabis use.

The only way to track this is through long-term studies which track cognition and brain development.

Vandrey is dismissive of the idea of residual impairment for occasional users. For a single use in a healthy adult, he thinks any thought that residual effects would linger for weeks are “complete nonsense”.

Because “disentangling” impairment from past use has been so hard, Vandrey says some employers in places where cannabis is legal have opted for a zero tolerance approach, particularly if a role is “safety sensitive”.

“A lot of workplaces have just said, we will not permit our employees to use cannabis, whether it’s legal or not. Pilots, bus drivers, highly sensitive construction jobs, the choice is you use cannabis legally and don’t work there, or you work there and don’t use it.”

He thinks if you’re using urine tests, it’s really a zero tolerance approach that you’re aiming for.

“You can’t differentiate the someone who uses cannabis on the weekend versus the person who smokes on the way into work. You just can’t. Especially with any kind of frequency of use. Urine has a really long window of detection.”

Opting for behavioural tests is an approach Vandrey is starting to see. A workplace can establish a baseline performance for an employee and then test periodically.

This could be through an app testing cognitive function, or even through goggles which look at eye sensitivity and function.

“There’s a lot of interest in developing those types of devices that measure impairment that’s really agnostic to drug … you don’t necessarily care if they’re impaired, whether it’s due to drinking, using cannabis, sleep deprivation, use of opioids or other impairing drugs, you only care that they’re impaired.”

Vandrey points out alcohol is an outlier in the drug world because it’s one of the few substances where testing for acute impairment is easy.

Closer to home Marta Rychert, from Massey University’s SHORE & Whariki Research Centre specialises in health, policy and law and has been working on topics related to cannabis law reform.

She also raises questions over the quality of scientific research available. Before people start quibbling over the window of impairment, she would like to see better data linking cannabis to workplace accidents.

She said a recent systematic review of workplace accidents and cannabis found 16 studies on the topic. Seven showed a link between cannabis use and workplace accidents, one study showed cannabis reduced accidents and the remaining eight studies showed no significant difference. Only three of the studies showed cannabis was consumed before a workplace injury and none of the studies could be sure workers were impaired at the time of the accident.

In the absence of good quality data, people instead draw parallels between workplace accident risk and the ability to drive a vehicle after using cannabis. Here, cannabis use is associated with a modest increase in risk of having an accident. It’s estimated people who have recently used cannabis are 1.5 to two times more likely to be involved in an accident.

Clemenger BBDO’s traffic safety campaign made for the NZ Transport Agency aimed to highlight slow reaction times when driving after using cannabis. Photo: Screenshot

Should legalisation go ahead in New Zealand, Rychert imagines some sectors will adopt different rules and some will opt for zero tolerance, or near zero tolerance. For example, in Canada, where cannabis was legalised in 2018, pilots are banned from using cannabis for 28 days before flying.

For now, Rychert thinks we’re stuck with making do with the types of tests available, such as saliva tests, until better tools are developed which track impairment.

“We need a breathalyser for THC, but that doesn’t exist yet.”

Without the invention of a THC breathalyser being imminent, The Drug Detection Agency’s Hardy is clear on his preferred testing mechanism for workplaces

He estimates saliva tests don’t identify 25 to 30 percent of people who may be at risk and at present no one can fully comply with new testing standards released in the past year. There are no laboratories accredited to deal with samples in New Zealand. He expects this will change over time. Saliva tests also don’t pick up edibles.

Behavioural tests like the modified Romberg Test, where participants stand on one leg, estimate thirty seconds and have their eye movement and pupil size tracked are often accurate he said, but impractical for workplaces because of the time it takes to conduct them.

He’s confident urine tests are the best test for safety-conscious workplaces.

“If you’re purely about safety in the workplace and identifying risks in the workplace; yes, definitely.”

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