The short answer
Review kava before combining it with promethazine because guidance advises against mixing kava with sedating substances. The exact pair’s added impairment risk has not been measured.
What this answer covers
Oral promethazine with the specific preparation described below.
- Supplement or preparation
- Piper methysticum
- Medication ingredient
- promethazine
The precaution concerns overlapping effects
Promethazine can impair alertness or performance. Kava guidance advises against combining it with other sedating substances, which supports a product-specific review rather than assuming the combination is harmless because one ingredient is botanical.
A small trial did not find standalone driving impairment
In a crossover study, twenty-two adults received kava, oxazepam and placebo in separate sessions. The tested kava dose did not impair simulated driving compared with placebo, whereas oxazepam affected selected measures. This is useful contrary evidence about that kava preparation.
Separate trial arms do not establish combination safety
Participants did not take kava together with promethazine in that trial. The result therefore cannot establish a safe combined dose or quantify impairment. Kava’s separate liver-injury concerns also do not prove that promethazine increases that risk.
Review the whole alertness burden
Bring the exact supplement label and all medicines to your pharmacist, particularly if promethazine already makes you sleepy, dizzy or confused. Avoid driving when affected. The reviewed evidence does not identify a supplement dose or hours-apart schedule that guarantees normal alertness.
Evidence and practical considerations
Side effects & toxicity
Review kava before combining it with promethazine because guidance advises against mixing kava with sedating substances. The exact pair’s added impairment risk has not been measured.
- Exact scope
- Oral promethazine with the specific preparation described below. with Oral promethazine; the hyoscine source, where relevant, is explicitly transdermal.
- Medication form and route
- oral
What remains uncertain
- Conditional impairment precaution follows kava guidance and exact medication labeling; standalone kava driving-null result retained. No exact pair incidence, measured respiratory depression or additive liver toxicity established.
Factors that may matter: Exact preparation and amount; Other medicines with overlapping effects; Route and gut-transit status where relevant.
Read the supporting source summaries
Source 1: Current U.S. regulatory product labeling
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: Patients prescribed oral promethazine.
Promethazine can cause marked drowsiness, can intensify other CNS depressants and requires caution with other anticholinergic medicines. It can lower the seizure threshold.
How this applies: Exact active ingredient and oral-route context.
Label warnings do not measure every exact supplement pair or provide a universal safe supplement dose or interval.
Source 2: Federal supplement guidance
nccih.nih.gov · Source check Sep 10, 2026
Population: People considering the named botanical or supplement.
Advises against combining kava with other sedating substances. Products vary and separate liver-injury concerns are described.
How this applies: Supports a sedation precaution, not a demonstrated exact-pair respiratory or liver injury rate.
General guidance does not quantify this exact medication combination or establish a universal protective interval.
Source 3: Original human study
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Twenty-two adults in a crossover driving-simulator trial.
Kava did not significantly impair driving compared with placebo; oxazepam impaired selected measures.
How this applies: Contrary standalone finding does not establish co-use safety.
Separate conditions, not simultaneous use. One product and dose, no target medication.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Initial source-checked research draft with exact preparation limits, contrary findings and source-access details. Clinical review not recorded. | |
| Research summary published |
Article revision: 869d947b8df5f1a8
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.