How long after smoking Tetrahydrocannabinol (THC) is an individual considered under the influence and what laboratory test confirms this?

Duration of THC Influence and Confirmatory Testing

The psychoactive effects of smoked THC last approximately 2-3 hours after inhalation, with peak impairment occurring 7-10 minutes after smoking and cognitive/motor effects returning to baseline within 3-5 hours. 1, 2

Timeline of Acute Impairment

Immediate Effects (0-10 Minutes)

• Peak blood THC concentrations occur 7-10 minutes after initiating smoking, reaching levels of approximately 152 ng/mL (±86.3) with a 3.55% THC cigarette 2, 3
• Psychoactive effects begin within seconds to minutes of inhalation 1
• Peak euphoria and maximal heart rate acceleration occur several minutes after peak blood concentrations, typically when smoking ends 3

Active Impairment Window (10 Minutes to 3-5 Hours)

• Physiological and behavioral effects return to baseline within 3-5 hours after a single dose 1
• The duration of measurable psychotropic effects after smoking a standard 9 mg THC cigarette is approximately 45 minutes, with recovery (decline to <50% maximum effect) lasting about 100 minutes after the last use 4
• Pharmacokinetic-pharmacodynamic modeling demonstrates that once equilibrium is reached, the intensity of effect is proportional to plasma THC levels 2

Critical Distinction: Impairment vs. Detection

• Blood THC levels decline rapidly after smoking ends, but this does not correlate linearly with impairment duration 5, 3
• In occasional users, serum THC may become undetectable within 4 hours, yet impairment can persist longer due to delayed equilibration with the brain 5

Confirmatory Testing for Active Impairment

Blood THC Testing (Gold Standard for Acute Use)

• Blood testing for THC itself (not metabolites) is the only method that can potentially indicate recent use and active impairment 6, 5
• Blood THC concentrations at the time of driving are substantially higher than at the time of delayed blood sampling, making interpretation challenging 5

Critical Testing Limitations

• Per se legal limits (2 ng/mL or 5 ng/mL THC) are scientifically problematic: 43% of regular cannabis users exceed zero-tolerance thresholds (≥0.5 ng/mL) after 48+ hours of abstinence, 24% exceed 2 ng/mL, and 5.3% exceed 5 ng/mL at baseline without recent use 7
• One participant showed 16.2 ng/mL after 48 hours of abstinence, demonstrating that blood THC levels alone cannot definitively prove recent use in chronic users 7
• Simulated driving performance showed no correlation with exceeding these legal cutpoints 7

Urine Testing (NOT Useful for Impairment)

• Urine tests detect 11-nor-9-carboxy-THC (THCCOOH), an inactive metabolite with a 6-day half-life 6
• Urine testing cannot distinguish between use 2 hours ago versus 2 weeks ago in regular users 6
• Detection windows: 1-3 days for occasional users, 30+ days for heavy users, up to 46-77 days in chronic heavy users 6

Practical Clinical Algorithm

For Assessing Current Impairment:

1. Time since last use is the most reliable indicator: If <3-5 hours since smoking, assume potential impairment 1, 2
2. Clinical assessment of cognitive and motor function is more reliable than blood levels alone 7
3. If blood testing is performed, interpret with extreme caution in known regular users, as baseline levels may exceed legal thresholds 7

For Chronic Heavy Users (Special Considerations):

• THC accumulates in fatty tissue with slow redistribution, causing persistent low-level blood concentrations 5, 2
• Chronic users may have permanent detectable THC levels, making blood testing unreliable for determining acute impairment 5
• These individuals show well-documented long-term adverse effects on neurocognitive function that persist beyond acute intoxication 5

Common Pitfalls to Avoid

• Do not rely on urine testing to determine if someone is currently impaired - it only indicates use within days to weeks 6
• Do not assume blood THC levels below legal thresholds mean no recent use - rapid redistribution can cause levels to drop quickly 5, 2
• Do not assume blood THC levels above legal thresholds prove impairment in chronic users - baseline levels may be elevated for days after cessation 7
• Do not use a single negative test to exclude cannabis use - timing relative to last use may result in undetectable levels 6