The short answer
St John’s wort extracts bound or precipitated some promethazine in laboratory mixtures, but reduced absorption or effectiveness in people has not been established. No protective spacing interval was tested.
What this answer covers
Oral promethazine with the specific preparation described below.
- Supplement or preparation
- Hypericum perforatum
- Medication ingredient
- promethazine
There is an exact laboratory experiment
Researchers mixed promethazine with aqueous St John’s wort preparations. Some medicine associated with particles or precipitated under the tested conditions. The amount depended on how the extract was prepared.
A mixture is not a human absorption study
The experiment did not give the combination to patients or measure promethazine blood levels, symptom control or adverse outcomes. What happens in a laboratory solution may not reproduce digestion, dilution and exposure in the body.
Do not substitute another drug’s interaction
St John’s wort has established interactions with several other medicines, but those findings do not supply a measured promethazine effect. The exact extract and the rest of the medication list deserve review, especially when starting or stopping the herb.
A timing rule would need separate support
This experiment did not test whether taking the products hours apart preserves treatment. Ask your pharmacist about the actual preparation rather than increasing promethazine or applying an untested gap to compensate for a theoretical absorption change.
Evidence and practical considerations
Absorption & effectiveness
St John’s wort extracts bound or precipitated some promethazine in laboratory mixtures, but reduced absorption or effectiveness in people has not been established. No protective spacing interval was tested.
- 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
- Direct in-vitro materials experiment is not a human pharmacokinetic or efficacy study. Infusion and reconstituted extract conditions differ; no clinical magnitude or validated gap.
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: Original laboratory study
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Laboratory mixtures of herbal extract and medicine.
Some promethazine bound to extract-associated particles or precipitated under tested conditions.
How this applies: Direct materials experiment supports a hypothesis without confirming clinical loss of effect.
Not a human absorption, efficacy or dosing-gap experiment. Laboratory concentration and preparation matter.
Source 3: Federal supplement guidance
nccih.nih.gov · Source check Sep 10, 2026
Population: People considering the named botanical or supplement.
St John’s wort has important interactions with certain medicines. Adult adverse effects include dizziness, restlessness and trouble sleeping.
How this applies: Does not establish adult carbinoxamine co-sedation; breastfeeding infant reports cannot be transferred to adult co-use.
General guidance does not quantify this exact medication combination or establish a universal protective interval.
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: e0dea35d18bb1f28
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.