The short answer
A promethazine-specific interaction with evening primrose oil has not been established. The historic seizure warning came from small, confounded reports involving other psychiatric medicines and a mixed supplement formula.
What this answer covers
Oral promethazine with the specific preparation described below.
- Supplement or preparation
- Evening Primrose Oil
- Medication ingredient
- promethazine
Where the older warning came from
An early schizophrenia study used evening primrose oil together with several vitamins and zinc. Three seizures were described, two during the active mixture and one during placebo. The active cases involved other antipsychotics; one patient had a prior seizure history and medication changes.
Promethazine was not the tested combination
The report did not identify promethazine use, so its events cannot establish this exact interaction. A later regulatory assessment critically discusses the old attribution. Neither source proves that evening primrose oil caused those seizures or guarantees that every combination is safe.
The medicine’s own seizure precaution still matters
Promethazine can lower the seizure threshold independently of evening primrose oil. A seizure history and other medicines that affect the threshold remain relevant to the prescriber’s decision, even when the supplement interaction itself is uncertain.
No protective supplement interval was established
Do not assume that separating evening primrose oil and promethazine by a few hours resolves a seizure concern. Bring the oil’s complete ingredient list to the pharmacist; the historical multi-ingredient formula does not establish a safe dose or an exact avoidance rule for pure oil.
Evidence and practical considerations
Side effects & toxicity
A promethazine-specific interaction with evening primrose oil has not been established. The historic seizure warning came from small, confounded reports involving other psychiatric medicines and a mixed supplement formula.
- 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
- Original trial contains no identified promethazine exposure, mixed nutrients, changing antipsychotics, prior seizure history and a placebo event. Current promethazine seizure-threshold warning remains relevant but does not validate a primrose-specific prohibition or interval.
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 human study
isom.ca · Source check Sep 10, 2026
Population: Twenty-three entered and thirteen remained for analysis in an old schizophrenia crossover study.
Three seizure events were described: two during active mixture treatment and one during placebo. Active cases had other antipsychotics and one had a prior seizure history.
How this applies: Original case source does not establish a promethazine-specific avoidance or timing rule.
No promethazine exposure identified. Multi-ingredient mixture, changing medicines, attrition and small numbers prevent isolation of primrose causality.
Source 3: Regulatory evidence assessment
ema.europa.eu · Source check Sep 10, 2026
Population: People considering covered Oenothera biennis/lamarckiana oil.
Critically discusses the weak historic seizure attribution and limited interaction reports.
How this applies: Supports cautious interpretation of historic phenothiazine generalization.
Assessment of earlier evidence, not a new promethazine clinical trial or a guarantee of safety.
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: 5255a6db4993247d
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.