EVIDENCE LIBRARY
How Rifampicin Changed THC and 11-Hydroxy-THC Peaks in a Crossover Study
A controlled drug-interaction study shows how THC and one of its metabolites can shift by different amounts.
When a laboratory measures THC and its metabolite 11-hydroxy-THC, the concentrations reflect both exposure and what happens after administration. A phase I study compared a THC/CBD mouth spray used alone with the same spray administered after repeated rifampicin in one group of healthy male volunteers.
The parent drug and metabolite shifted differently
After rifampicin, the reported peak plasma concentration of THC fell from 2.94 to 1.88 ng/mL. The reported peak for 11-hydroxy-THC fell from 3.38 to 0.45 ng/mL. The crossover design allowed the researchers to compare conditions within the same participants.
The peaks represent the highest measured concentrations for each compound and can occur at different times. Dividing one peak by the other would therefore give a ratio of two summary values. Applying that ratio to an individual blood specimen would require data on the timing of both measurements and the conditions under which the specimen was collected.