Air India pilot's reported cannabis test: What it can and can't tell investigators

A review conducted by aviation‑medicine specialist David Newman for Australia’s Transport Safety Bureau reached a comparable conclusion. Cannabis influences behaviour, cognition and psychomotor performance in a dose‑dependent manner, and complex tasks such as flying are especially sensitive to those effects. The review also identified evidence of residual impairment that can persist for up to 24 hours. The issue has grown more complicated as cannabis use has become more widespread and the potency of products has risen in many regions of the world. In 2022, a randomised objective trial led by Thomas Marcotte at the University of California San Diego examined 191 regular cannabis users. Participants who smoked THC performed worse on a simulated driving task, yet the researchers determined that impairment could not be inferred from the THC concentration in the cannabis, the users’ prior experience, or their blood THC levels. Although the drug initially impaired performance, after roughly four‑and‑a‑half hours those who had consumed it were performing at a level comparable to participants who had not received the active compound. Marcotte emphasized the relevance of this finding for aviation testing. “A positive test can show that THC is present, but not necessarily when cannabis was consumed or whether the individual was impaired at the time,” he said. He added that the relationship between THC and impairment varies with dose, route of administration, frequency of use and individual biology. “Unlike with alcohol where there’s a much more robust relationship between alcohol concentrations in blood and impairment, the relation is much more complex with THC,” Marcotte told the BBC. “THC rapidly leaves the bloodstream after smoking, and there is limited to no correlation between blood THC concentrations and impairment.” He also noted, “In people who use at least occasionally, THC can be detectable in blood for days and sometimes weeks after past use, particularly in more frequent users. This is long after any acute impairment has subsided.” Research indicates that impairment after smoking cannabis typically lasts three to five hours. When cannabis is ingested—as in an edible—impairment endures longer, generally six to eight hours and occasionally more. After edible consumption, the metabolite 11‑OH‑THC (11‑hydroxy‑THC) is often more closely linked to impaired cognitive function than THC itself, which may be present only at low concentrations, according to Marcotte. Consequently, no universally accepted cannabis test exists that can definitively state that a person is presently too impaired to fly. Canada has adopted a deliberately blunt stance. Following the legalisation of recreational cannabis in 2018, Transport Canada introduced a rule prohibiting flight crews and flight controllers from using cannabis for at least 28 days before duty. The regulator says the regulation was intended to address the uncertainties surrounding cannabis detection and impairment, and it allows airlines to impose stricter standards if they choose. The U.S. aviation experience offers a different caution against over‑interpreting a positive result. A 2026 National Transportation Safety Board study of pilots killed in U.S. civilian aviation accidents between 2018 and 2022 found that 52.8 % had at least one drug detected, while 27.7 % had two or more substances present. THC was the primary contributor to the rise in illicit‑drug detections. The NTSB stressed that the analysis concerned drug presence only, not whether the pilots were impaired at the time of the accidents.