NutriStack
Cannabis (THC-Dominant) and Promethazine, a caution.
THC-dominant cannabis can add to promethazine-related sedation, slowed reactions, and impaired coordination. Promethazine impairs psychomotor function in controlled human studies, while THC acutely impairs driving-related performance. The combination is especially concerning with nausea/cough products that also contain opioids or other sedatives.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
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At a glance
- Substances
- Cannabis (THC-Dominant) and Promethazine
- Pair type
- Caution
- Evidence (highest tier)
- Moderate
- Source citations
- 3 sources
- Stack Score effect
- −5 to your Stack Score (per scored caution row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Caution · Moderate evidence
Caution
What is happening. THC-dominant cannabis can add to promethazine-related sedation, slowed reactions, and impaired coordination. Promethazine impairs psychomotor function in controlled human studies, while THC acutely impairs driving-related performance. The combination is especially concerning with nausea/cough products that also contain opioids or other sedatives.
Mechanism. Promethazine produces CNS depression through H1 blockade plus antimuscarinic and antidopaminergic effects. THC activates CB1 receptors and impairs attention, tracking, and reaction time, creating additive psychomotor and judgment impairment.
Recommendation. Do not combine THC-dominant cannabis with promethazine before driving or any safety-sensitive task. If both were used, stay in a safe setting and avoid adding alcohol, opioids, or sleep medicines. Use the lowest effective promethazine dose only as prescribed.
Sources (3)
- Kocis PT, Vrana KE. Delta-9-Tetrahydrocannabinol and Cannabidiol Drug-Drug Interactions. Med Cannabis Cannabinoids. 2020;3(1):61-73. PMID 34676340
- Kamei H, Isaji A, Noda Y, et al. Effects of single therapeutic doses of promethazine, fexofenadine and olopatadine on psychomotor function and histamine-induced wheal- and flare-responses: a randomized double-blind, placebo-controlled study in healthy volunteers. Arch Dermatol Res. 2012;304(4):263-272. PMID 22130869
- Simmons SM, Caird JK, Sterzer F, Asbridge M. The effects of cannabis and alcohol on driving performance and driver behaviour: a systematic review and meta-analysis. Addiction. 2022;117(7):1843-1856. PMID 35083810
Stack Score
How this pair moves the number.
Effect on the composite score
If both Cannabis (THC-Dominant) and Promethazine are in the same stack, this pair applies −5 to your Stack Score (per scored caution row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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