Cannabis Use — THC Enters the Bloodstream
THC absorbs quickly after smoking or vaping — blood level spikes within minutes of use, long before actual driving impairment peaks.
- ~10 min: Time to peak THC (smoked route)
- 10–35%: Bioavailability (smoked route)
- Seconds: Onset of subjective effects
- ~1 hr: Initial distribution-phase half-life
Absorption and distribution
THC is lipophilic: it absorbs fast into the blood, then redistributes into fat tissue, which is why blood level falls sharply after the first hour even while THC keeps circulating at low levels for days.
Driving-Relevant Skills Decline at Peak THC
Reaction time, divided attention, and lane tracking all worsen in the hour or two after use, roughly tracking blood level during this early window.
- ~20%: Reaction time slowdown near peak
- Notable: Lane deviation increase in simulator studies
- Impaired: Divided-attention (dual-task) performance
- 1–4 hr: Typical impairment window
Which skills degrade
Attention-splitting and continuous tracking tasks — exactly what driving demands — suffer more than simple reaction tests.
Blood Level No Longer Predicts Impairment Well
Unlike blood alcohol concentration, THC level and actual impairment drift apart after the first couple of hours.
- Weak: THC-impairment correlation post-peak
- Strong: Alcohol BAC-impairment correlation throughout
- Variable: Tolerance effect in frequent users
- High: Individual metabolic variability
Why the link breaks down
Fat storage releases THC slowly back into blood long after the intoxicating effect has faded, and regular users build tolerance that further decouples measured level from functional impairment.
THC Detectable Long After Impairment Ends
Metabolites linger in blood and urine for days to weeks — the "detection gap" this simulator visualizes.
- ~4–8 hr: Typical impairment resolution
- Days: Blood detection window (longer for frequent users)
- Weeks: Urine detection window for frequent users
- Hours: Alcohol's detection window, which matches its impairment window
The long metabolite tail
THC-COOH persists in fat tissue and is released slowly over time, which is why a test days later can still be positive even though driving skill returned to normal within hours.
Why This Complicates Impaired-Driving Law
A positive THC test does not confirm current impairment, unlike a breathalyzer's tight link to alcohol impairment.
- Elevated: False-positive-for-impairment risk on post-use testing
- Contested: Legal per-se THC limits vary by jurisdiction
- Limited: Field sobriety test value for cannabis specifically
- Reliable: Alcohol's BAC standard, by comparison
Policy implications
Because detection outlasts impairment, some jurisdictions are exploring behavioral and performance-based testing alongside or instead of a fixed THC blood threshold.
Detection is not the same as impairment.