Diphenhydramine
| Evidence Level: L5 | Predicted Indications: 10 |
Table of Contents
Diphenhydramine: From Allergic Rhinitis and Insomnia to Rosacea Conjunctivitis
One-Sentence Summary
Diphenhydramine is a first-generation H1 antihistamine, originally used for allergic and vasomotor rhinitis, allergic conjunctivitis (atopic, vernal and papillary) and insomnia. The TxGNN model's top-ranked prediction is rosacea conjunctivitis, but no clinical trials or publications support it, so it rests on model prediction alone. The better-supported candidates in the list are rhinitis (an on-label or near-label use, backed by 5 trials and 18 publications) and allergic urticaria.
Quick Overview
| Item | Content |
|---|---|
| Original Indication | Insomnia, vasomotor rhinitis, allergic rhinitis, atopic, papillary and vernal conjunctivitis (the registration records compiled for this report contain no approved indication text) |
| Predicted New Indication | Rosacea conjunctivitis |
| TxGNN Prediction Score | 99.20% |
| Evidence Level | L5 |
| Singapore Market Status | ✓ Marketed |
| Number of Registrations | 14 |
| Recommended Decision | Hold |
Note: The top-ranked prediction has no supporting evidence. Among the 10 predictions, rhinitis has the strongest evidence (Level L1, Proceed with Guardrails), but it largely overlaps with an existing indication. It is covered in the sections below as the most actionable direction.
Why is This Prediction Reasonable?
Currently, detailed mechanism of action data is not available in the data compiled for this report. Based on known pharmacology, diphenhydramine blocks the histamine H1 receptor and has anticholinergic and sedative effects. Its efficacy in allergic conjunctivitis and allergic rhinitis is established.
The model's high score for rosacea conjunctivitis probably reflects its similarity to the approved allergic eye indications and to H1 antagonism. However, rosacea-associated eye disease is driven mainly by meibomian gland dysfunction and inflammation rather than histamine, so the mechanistic rationale is weak. No trials or literature were found.
For the other predictions:
- Rhinitis: H1 blockade reduces sneezing, rhinorrhoea and itching. This is effectively an existing use rather than true repurposing.
- Allergic urticaria and cold urticaria: Histamine release from mast cells drives wheals and itch, so H1 blockade is plausible. Newer-generation antihistamines are preferred first-line for safety reasons.
- Cauda equina syndrome, viral conjunctivitis, glossodynia: No plausible mechanistic link, or only speculative; these look like graph artefacts.
- Neuralgia, trigeminal autonomic cephalalgia, nasopharyngitis: Indirect or historical evidence only.
Clinical Trial Evidence
No clinical trials are registered for rosacea conjunctivitis. Trials for the best-supported candidate, rhinitis, are listed below for reference.
| Trial Number | Phase | Status | Enrollment | Key Findings |
|---|---|---|---|---|
| NCT00648973 | Phase 4 | Completed | 1021 | Diphenhydramine 25 mg and 50 mg vs placebo and pseudoephedrine for nasal congestion in seasonal allergic rhinitis |
| NCT00762749 | Phase 1 | Completed | 36 | Single-dose pharmacokinetics in children and adolescents (supports dosing, not efficacy) |
| NCT01177852 | Phase 3 | Withdrawn | 0 | Fixed-dose diphenhydramine combination in children with cough and acute rhinitis; no data |
| NCT05586477 | Phase 4 | Completed | 20 | Effect on thermoregulation during exercise (not an efficacy study) |
| NCT06217367 | Phase 4 | Unknown | 16 | Effect of OTC antihistamines on thermoregulation during heat stress (not an efficacy study) |
Literature Evidence
No literature was found for rosacea conjunctivitis. Selected publications for rhinitis are listed below.
| PMID | Year | Type | Journal | Key Findings |
|---|---|---|---|---|
| 16680933 | 2006 | RCT | Ann Allergy Asthma Immunol | Diphenhydramine vs desloratadine vs placebo in moderate-to-severe seasonal allergic rhinitis |
| 14582817 | 2003 | RCT | Ann Allergy Asthma Immunol | Diphenhydramine vs desloratadine on vigilance and cognition in ragweed allergic rhinitis |
| 8634878 | 1996 | RCT | Ann Allergy Asthma Immunol | Effects on learning in young adults with seasonal allergic rhinitis |
| 12113226 | 2002 | Review | Clin Allergy Immunol | H1-antagonists are useful in allergic rhinoconjunctivitis in children; not supported for upper respiratory infections |
| 31582993 | 2019 | Position statement | Allergy Asthma Clin Immunol | Newer-generation antihistamines are safer and should be first-line over first-generation agents |
| 16278258 | 2005 | Review | Ann Pharmacother | Efficacy and safety of oral antihistamines in allergic rhinitis and chronic urticaria |
| 36420548 | 2022 | Pilot study | Tokai J Exp Clin Med | Transdermal diphenhydramine applied to the nasal ala in allergic rhinitis with asthma |
| 36759413 | 2023 | Preclinical | AAPS PharmSciTech | Nasal nano-gel and nano-emulgel in a rabbit allergic rhinitis model |
| 40152721 | 2025 | Review | Med Lett Drugs Ther | Treatment of allergic rhinitis and allergic conjunctivitis |
Singapore Market Information
| Authorization Number | Product Name | Dosage Form | Approved Indication |
|---|---|---|---|
| SIN00362P | BENOCTEN TABLET 50 mg | Tablet | Not stated in the registration record |
| SIN10851P | PAXIDORM TABLETS 25 mg | Sugar-coated tablet | Not stated in the registration record |
| SIN02839P | BEANAMINE EXPECTORANT | Solution | Not stated in the registration record |
| SIN08782P | DIPHEN 5 EXPECTORANT | Syrup | Not stated in the registration record |
| SIN08799P | DIPHEN 10 EXPECTORANT | Syrup | Not stated in the registration record |
There are 14 valid single-ingredient licences in total (tablet, sugar-coated tablet, solution, syrup and elixir forms). Most are expectorant syrups.
Safety Considerations
- Drug Interactions: 100 interactions are recorded. Two Major ones are shown in the sample reviewed: potassium citrate and eliglustat. Many Moderate ones involve anticholinergics (hyoscyamine, atropine, glycopyrronium, dicyclomine) and CNS depressants or opioids (morphine, nabilone, dronabinol). Caution is needed with CNS depressants and anticholinergics.
- Class Effects (from the literature): Sedation, cognitive impairment, dry mouth, dizziness and orthostatic hypotension, especially in children and older adults. Rapid IV administration or large doses may cause toxicity.
Package insert warnings and contraindications were not available for this report. Please refer to the package insert for full safety information.
Conclusion and Next Steps
Decision: Hold (for the top-ranked prediction, rosacea conjunctivitis)
Rationale: The model score is high (99.20%), but there are no trials or publications, and the mechanism (meibomian gland dysfunction and inflammation) is not histamine-driven. The evidence level is L5.
Other candidates:
- Rhinitis: Level L1, Proceed with Guardrails. It is effectively an existing indication, so it is not a true repurposing opportunity. Guardrails are sedation, anticholinergic effects and cognitive impairment, plus DDI caution.
- Allergic urticaria: Research question (L4). The only Phase 3 trial tests IV cetirizine, not diphenhydramine.
- Cold urticaria, nasopharyngitis, neuralgia, trigeminal autonomic cephalalgia: Hold. Evidence is indirect, historical or case-level.
- Cauda equina syndrome, viral conjunctivitis, glossodynia: Hold. No plausible link or no evidence.
To proceed, the following is needed:
- Singapore package insert warnings, contraindications and approved indication text (a blocking gap for safety screening)
- Detailed mechanism of action data
- For rosacea conjunctivitis: any clinical or mechanistic evidence of benefit, plus a review of whether the prediction is an artefact of the allergic conjunctivitis indications
- For urticaria: head-to-head data against second-generation antihistamines
- Route and formulation compatibility assessment for the target indication
Disclaimer
This content is for research purposes only and does not constitute medical advice. Clinical validation is required before any clinical application.