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A review , externalby aviation-medicine specialist David Newman for Australia's Transport Safety Bureau reached a similar conclusion.
Cannabis affects behaviour, cognition and psychomotor performance in a dose-dependent way, and complex tasks such as flying are particularly sensitive. The review also found evidence of residual effects lasting up to 24 hours.
The science has become more complicated, however, as cannabis use has become more widespread and the potency of cannabis products has increased in many parts of the world.
A 2022 randomised clinical trial, external led by Thomas Marcotte at the University of California San Diego tested 191 regular cannabis users.
Those who smoked THC performed worse on a simulated driving task, but the researchers found that impairment could not be inferred from the THC content of the cannabis, the users' previous experience or their blood THC concentration.
The drug initially impaired performance, but after about four-and-a-half hours, those who had taken it were performing about as well as people who had not received the active drug.
That finding matters enormously for aviation testing.
A positive test can show that THC is present, but not necessarily when cannabis was consumed or whether the person was impaired at the time. The relationship varies with the dose, how it was consumed, frequency of use and individual biology, experts say.
This is one reason cannabis presents regulators with a problem that alcohol does not.
"Unlike with alcohol where there's a more robust relationship between alcohol concentrations in blood and impairment, the association is much more complex with THC. THC rapidly leaves the bloodstream after smoking, and there is limited to no correlation between blood THC concentrations and impairment," Marcotte told the BBC.
"In people who use at least occasionally, THC can be detectable in blood for days and sometimes weeks after last use, particularly in more frequent users. This is long after any acute impairment has subsided."
Studies show that impairment after smoking cannabis typically lasts three to five hours. When cannabis is ingested, such as in an edible, impairment lasts longer - typically six to eight hours, and sometimes more.
After edible use, the metabolite 11-OH-THC (11-hydroxy-THC) is often more closely associated with impaired cognitive function than THC itself, which may be present at lower concentrations, says Marcotte.
Not surprisingly, there is no equivalent universally accepted cannabis test that can say: this person is currently too impaired to fly.
Canada has responded by taking a deliberately blunt approach. After legalising recreational cannabis in 2018, Transport Canada introduced a rule, external prohibiting flight crews and flight controllers from consuming cannabis for at least 28 days before duty.
The regulator says the rule was intended to address the uncertainties surrounding cannabis impairment and detection; and airlines can impose stricter rules.
The American aviation experience offers another warning against reading too much into a positive test.
A 2026 US National Transportation Safety Board (NTSB) study , externalof pilots killed in US civil aviation accidents between 2018 and 2022 found that 52.8% had at least one drug detected, while 27.7% had two or more. THC was the main driver of the increase in illicit-drug detections. But the NTSB stressed that the study was about drug presence, not whether pilots were impaired.
