Cannabis, Bicycle Performance, and Blood THC: What One Controlled Study Found

A blood THC result can establish that THC was present in the specimen. It does not automatically establish how a person performed on a bicycle, whether the person was unable to ride safely, or when any effect occurred.

A 2016 study in the International Journal of Legal Medicine tested fourteen regular cannabis consumers on a controlled bicycle course before and after they smoked cannabis. Although serum THC concentrations reached as high as 117 ng/mL, the investigators observed few riding faults and could not identify a serum THC concentration that defined inability to ride.

That finding is important, but its limits are equally important. The study was small, involved regular consumers, used a closed course, and included participants with additional drug findings. It is evidence against treating a THC number as a stand-alone performance verdict. It is not proof that cannabis can never impair cycling.

What the researchers actually tested

The study enrolled fourteen regular cannabis consumers, twelve men and two women, with a median age of twenty-five. Reported cannabis-use patterns ranged from approximately one gram per week to one gram per day.

Each participant first became familiar with a protected, non-public bicycle course and then completed a baseline series. Afterward, the participants could smoke as many as three body-weight-adjusted cannabis cigarettes. Each cigarette contained 300 micrograms of THC per kilogram of body weight.

The participants completed repeated rides shortly after cannabis consumption and again about two hours after consumption ended. The course included straight-line riding, slalom elements, progressively narrower spacing, a traffic-light reaction task, stop lines, a memory task, unexpected obstructions, glare, verbal distraction, and movable barrels. The researchers also performed medical and ophthalmological examinations and collected serial blood samples.

This was a performance experiment with measured concentrations, not a study that inferred performance from a laboratory result alone.

The measured THC concentrations covered a wide range

Several participants arrived with measurable serum THC before the study cannabis was administered. Initial concentrations ranged from below the analytical limit of detection to 9.4 ng/mL. After cannabis consumption, measured serum THC concentrations reached 117 ng/mL.

The researchers grouped observations into serum THC ranges below 5 ng/mL, 5 to 15 ng/mL, and above 15 ng/mL. They also evaluated a calculated cannabis influence factor, or CIF, intended to reflect recent exposure using THC and metabolite concentrations.

The concentration groups did not produce a clean progression of riding errors. Some participants with serum THC above 100 ng/mL rode no more suspiciously than they had during baseline or than other participants did before the study cannabis.

That mismatch is the central forensic point: a concentration and a demonstrated functional deficit are not interchangeable measurements.

For the timing and movement of drugs through the body, see Pharmacokinetics in Toxicology and Drug Distribution in Toxicology.

Riding errors did not rise with the THC groups

The investigators scored coordinative mistakes, concentration-related mistakes, and responses to complex situations. Baseline performance for each participant was also used as that participant's comparison for later rides.

The median total demerits did not increase after cannabis consumption. The analysis found no significant difference among the three serum THC groups for overall riding faults. Relative riding performance remained nearly constant at the group level.

Severe coordinative errors occurred in some post-consumption rides, but they did not line up in a simple dose-response pattern. The investigators therefore did not propose either a serum THC threshold or a CIF threshold that established inability to ride a bicycle.

This does not turn an absence of group separation into proof of no effect. It shows that this experiment did not support using the measured THC concentration as a reliable stand-alone classifier of riding inability.

Medical observations and riding performance answered different questions

The researchers reported more observable features on medical examination after cannabis use than they observed faults on the bicycle course. The median amplitude of fusion, an ophthalmological measure related to binocular vision, decreased immediately after consumption and later increased as concentrations fell, although the reported change was not statistically significant.

An examination sign may support an inference about recent exposure or a possible drug effect. It does not necessarily demonstrate that the person committed a dangerous riding error. Conversely, acceptable performance on one controlled course does not establish that every real-world traffic task would be unaffected.

The laboratory result, medical examination, and observed performance are separate evidence streams. They should be compared, not collapsed into one conclusion.

The study had substantial limitations

The authors identified several limitations that matter in a forensic review:

• Only fourteen participants were studied.
• Twelve participants were men, limiting the ability to examine sex-related differences.
• All were regular cannabis consumers, so the results may not apply to infrequent or inexperienced users.
• Most participants had measurable cannabinoids before study dosing.
• Four participants later had amphetamine and/or MDMA findings, despite initial urine screening that did not reveal other drugs.
• Two participants had concentrations of those stimulants considered potentially active and were marked separately in the figures.
• The tests occurred on a protected course, not in uncontrolled public traffic.
• The reported p values were not adjusted for multiple comparisons.
• The scoring system and course measure selected riding behaviors, not every component of perception, judgment, divided attention, or crash avoidance.

Those limitations do not erase the observed data. They constrain the question the data can answer.

Regular use and tolerance cannot be ignored

The participants' regular cannabis use is not a minor detail. Repeated exposure can alter the relationship among dose, blood concentration, subjective effects, and measured performance. A result obtained in regular consumers cannot automatically be assigned to an occasional consumer.

Case interpretation should therefore identify the person's use history, claimed tolerance, dose, product strength, route, and time since use. None of those facts can be reconstructed reliably from a serum THC number alone.

For the difference between measured parent compounds and metabolites, see Cannabis Metabolites in Toxicology.

Serum is not whole blood

The Hartung study reported serum concentrations. Many forensic laboratories report cannabinoids in whole blood. Those matrices are not numerically interchangeable.

A comparison that treats a serum result and a whole-blood result as though they were the same can distort the analysis. Before using the study in a case, identify the matrix, collection tube, collection time, storage conditions, analytical method, reporting basis, and whether any conversion was performed.

See Whole Blood and Plasma in Toxicology and Biological Matrices in Toxicology.

Records needed for a case-specific evaluation

A defensible review should obtain:

1. The exact riding conduct, traffic conditions, roadway design, lighting, weather, and any collision evidence.
2. All video, witness observations, dispatch recordings, and contemporaneous notes.
3. The time of alleged riding, stop, examination, blood draw, and every reported cannabis-use event.
4. The product, THC content, amount, route, and pattern of use.
5. The person's cannabis-use history, medical use, claimed tolerance, sleep, fatigue, illness, injury, and vision evidence.
6. The complete toxicology report, matrix, method, calibrators, controls, chromatograms, reporting limits, and measurement uncertainty.
7. Results for THC, 11-OH-THC, THC-COOH, alcohol, medications, and other drugs.
8. The complete examination protocol and the examiner's training, instructions, scoring, and deviations.
9. The factual and scientific basis for comparing the individual with fourteen regular consumers on a closed course.
10. Any opinion's treatment of the study's stimulant findings, baseline cannabinoid concentrations, small sample, and unadjusted multiple testing.

See Measurement Uncertainty in Toxicology for the difference between a reported number and the certainty of an inference drawn from it.

The bottom line

In this controlled experiment, fourteen regular cannabis consumers completed repeated bicycle tests before and after cannabis use. Serum THC reached 117 ng/mL, yet overall riding faults did not increase across the concentration groups, and the investigators could not identify a serum THC concentration that defined inability to ride.

The study does not prove that cannabis never affects cycling. It shows why a serum THC concentration, without the person's timing, matrix, use history, observed conduct, examination, and complete toxicology record, is not a stand-alone measure of riding ability.

Primary source

Hartung B, Schwender H, Roth EH, et al. The effect of cannabis on regular cannabis consumers' ability to ride a bicycle. International Journal of Legal Medicine. 2016;130(3):711-721. PubMed record. doi:10.1007/s00414-015-1307-y.

Editorial note: This article paraphrases the published research. Do not upload or reproduce the source PDF, publisher layout, tables, or figures.