Does Huperzine A interact with Promethazine hydrochloride?

Medication · antihistamine

Supplements and Promethazine hydrochloride.

Every documented pair, every citation. Below: 1 documented pair grouped by mechanism.

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 amber. 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 is assessed against PubMed-indexed evidence.

Cholinergic vs anticholinergic (opposing)

Amber Does Huperzine A interact with ?

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: Based on an established mechanism, with no direct study of this exact combination yet.

PMID 29350336 · PMID 18425924 · BNF: Promethazine

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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