CBD’s Terminal Half-Life Did Not Predict Its Accumulation Rate in the Same Way

forensic toxicology working group members October 28, 2026 Forensic Toxicology

A long terminal CBD half-life did not mean plasma accumulation followed that same time scale. A phase I study distinguished the slow terminal tail from an effective half-life derived from repeated dosing, and a correction repaired the printed equation.

Different regimens supplied different information

Healthy volunteers received single ascending doses, repeated twice-daily CBD or a fed-versus-fasted comparison. Researchers measured CBD and metabolites in plasma. Repeated-dose groups received 750 or 1,500 mg twice daily during the dosing period.

After repeated dosing, terminal CBD half-life was approximately 56–61 hours. Trough observations nevertheless suggested parent CBD approached steady state in about two days. Those observations can coexist with a multiphasic decline.

A terminal tail is not the whole curve

Terminal half-life describes the final measured decline after dosing stops. If that phase contributes little to total exposure, it may poorly describe accumulation during regular dosing.

The authors discussed an effective half-life incorporating dosing interval and accumulation ratio, with estimates of approximately 10–17 hours. This describes the studied regimens, not a duration of intoxication or negative-test deadline.

The corrected equation matters

The 2019 correction replaced an erroneous leading term in the effective-half-life equation with the dosing interval, represented by tau. Reusing the original printed equation would misstate the calculation.

The distinction is not permission to select whichever half-life favors a desired argument. Specimen, sampling period, elimination phase and purpose determine which parameter is relevant.

Limits of translation

The study was short and used a particular purified oral formulation in healthy volunteers. Gastrointestinal and nervous-system adverse events occurred despite the absence of severe or serious adverse events.

It did not establish long-term safety, therapeutic benefit or individualized washout time. Different sampling durations in the single- and repeated-dose arms further emphasize why half-life requires experimental context.

For the separate forensic question, see our discussion of cannabinoid concentrations and evidence of impairment.