High-Dose Oral CBD Increased THC Exposure in a Trial That Also Administered Probe Medications

Adding a large oral CBD dose did not protect participants from THC effects in this crossover trial. It increased cannabinoid exposure and several effects, but the accompanying medication cocktail is essential to interpreting the experiment.

The same THC dose, different CBD content

Eighteen healthy adults completed randomized, blinded sessions with placebo, a 20-mg THC brownie or a brownie containing 20 mg THC plus 640 mg CBD. Thirty minutes later, participants received a cytochrome P450 probe-drug cocktail.

The cocktail contained caffeine, omeprazole, losartan, dextromethorphan and midazolam. It was included to investigate drug metabolism. This was not simply two retail edibles taken without other substances.

Greater exposure accompanied stronger effects

The THC-plus-CBD condition produced greater plasma exposure to THC and its measured metabolites than THC alone. Several subjective and cognitive or psychomotor effects were also greater with the combination.

The pattern was not identical for every outcome. An increase in unpleasant effects does not mean every pleasurable rating, task score or vital sign significantly differed between active conditions.

A plausible interaction is not a universal rule

The findings are consistent with CBD changing the disposition of oral THC, for which intestinal and hepatic metabolism matter. They do not establish that every CBD dose or ratio produces the same interaction.

There was no CBD-only arm, and the trial used one THC dose and one large CBD dose. The probe drugs may have contributed to behavioral outcomes. Their presence in every condition does not eliminate possible interactions in the active cannabis sessions.

What can reasonably be inferred

CBD content should not be assumed to neutralize oral THC. Route, absolute doses, formulation and other medications need to be identified before comparing exposures.

The small sample was not designed to settle every sex difference or estimate crash risk. Plasma concentrations and laboratory responses do not supply a universal whole-blood threshold or individualized driving clearance.

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

Primary research

Assessment of Orally Administered Δ9-Tetrahydrocannabinol When Coadministered With Cannabidiol on Δ9-Tetrahydrocannabinol Pharmacokinetics and Pharmacodynamics in Healthy Adults: A Randomized Clinical Trial. JAMA Network Open (2023). DOI: 10.1001/jamanetworkopen.2022.54752.