Supplements and Promethazine hydrochloride.
Promethazine hydrochloride, sold under the brand names Phenergan, Sominex, Avomine, is an antihistamine.
Promethazine hydrochloride is an antihistamine. The class divides into two generations. Older agents (chlorphenamine, promethazine, diphenhydramine) cross the blood brain barrier and cause sedation. Newer agents (cetirizine, loratadine, fexofenadine) mostly do not. UK community prescribing is mostly the newer agents for allergic rhinitis and urticaria. The supplement surface for them is small, because they have low CYP involvement and minimal additive sedation effect. The older agents stack with anything else sedating (alcohol, benzodiazepines, valerian, kava, magnesium glycinate at higher doses) and with anticholinergic supplements. The latter matters in older patients, given the cognitive decline signal in data on sustained anticholinergic burden. Fexofenadine specifically is a P-glycoprotein substrate and shows reduced absorption with fibre supplements at high intake, and with fruit juices. The BNF flag is real, but the clinical magnitude is small at standard doses.
Below are the 1 documented pair we have explicitly assessed against Promethazine hydrochloride in the Distil database: 1 caution. The pairs cluster around 1 mechanism: Cholinergic vs anticholinergic (opposing). Every call is cited to either a clinical reference (PMID) or the British National Formulary. Anything not on this list is either still to be assessed or beyond our database scope. The checker beneath surfaces assessments by supplement, and the missing-item form at the bottom of the page routes any uncatalogued supplement into our next curation pass.
Documented interactions
Last reviewed 15 June 2026. Each interaction below lists the evidence its assessment drew on: some rest on studies of the exact pair, others on mechanism evidence, where a citation may support one part of the reasoning rather than the exact combination. The evidence line on each entry says which.
Cholinergic vs anticholinergic (opposing)
Caution Does Huperzine A interact with Promethazine hydrochloride?
Huperzine A raises acetylcholine, while promethazine (a sedating antihistamine) has an anticholinergic effect that blocks it. The two can work against each other, so promethazine may blunt any benefit from huperzine A. It is worth mentioning the huperzine A to your prescriber.
Evidence: No direct study of this exact combination yet; the caution rests on a plausible mechanism or on reports involving a related medicine.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
What this list does not say. Pairs not flagged here are not implicitly safe. They are either not yet in our database, or fall outside our inclusion scope (food-supplement interactions only; for drug-drug interactions, the BNF is authoritative). Use the checker below to surface any supplement, and submit a missing item if you take something we have not catalogued.
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How the checker decides
Every result has a source
Each pair we have assessed is cited to a published study (PMID) or the British National Formulary, so you can read the source yourself.
Four kinds of answer
Severe, caution, no known interaction, and not yet assessed. A pair we have not assessed is never shown as clear.
- Severe
- Caution
- No known interaction
- Not yet assessed
Nothing in our data is not a clearance
When nothing in our data records a problem with a pair, the result says no known interaction at typical doses, which is not a clearance: your pharmacist or GP can check the pair against your full medical history.
A full supplement review, built around you, not a generic stack.
The free checker answers one interaction question at a time. A Distil report reviews the bigger picture: your goals, diet, medications, current supplements and any blood results you provide, then shows what appears worth keeping, changing, adding or leaving out.
- what your diet already covers, so you are not adding a capsule for something food is already handling
- which evidence-graded compounds are worth considering for your goals, and which were considered but left out
- what to keep, change or stop from what you already take, plus the order to introduce anything new
- how your medications and complete supplement stack fit together, with daily timing and a clear point for reassessment
The interaction check is one section of the full report. You also get the reasoning behind each recommendation and the evidence references used.
£49 until 30 June 2027 · £79 from 1 July 2027.
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