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    Diphenhydramine molecular structure

    Diphenhydramine Stats & Data

    Depressant Psychedelic Deliriant
    Dph Nytol Sominex Zzzquil Benylin benadryl
    NPS DataHub
    MW255.36
    FormulaC17H21NO
    CAS58-73-1
    IUPAC2-benzhydryloxy-N,N-dimethylethanamine
    SMILESCN(C)CCOC(c1ccccc1)c1ccccc1
    InChIKeyZZVUWRFHKOJYTH-UHFFFAOYSA-N
    Psychoactive Class Depressant / Psychedelic

    Pharmacology

    DrugBank
    State Solid

    Description

    Diphenhydramine - perhaps known most commonly as its brand name formulation Benadryl - is a first-generation H1 receptor antihistamine that is used extensively for the treatment of seasonal allergies, insect bites and stings, and rashes . However, it also has antiemetic, antitussive, hypnotic, and antiparkinson properties . As histamine receptors exist both peripherally and in the central nervous system, diphenhydramine has been shown to cause sedation due to its competitive antagonism of histamine H1 receptors within the central nervous system . While its use in allergy therapy can sometimes fall out of favor due to its sedative effect, diphenhydramine has been repurposed for use within many non-prescription over-the-counter sleep aids and cough-and-cold medications that have been marketed for "night time" use . Diphenhydramine is also used in combination with DB14132 as the anti-nausea drug DB00985 where it is utilized primarily for its antagonism of H1 histamine receptors within the vestibular system . Diphenhydramine has also been shown to be implicated in a number of neurotransmitter systems that affect behaviour including dopamine, norepinephrine, serotonin, acetylcholine, and opioid . As a result, diphenhydramine is being investigated for its anxiolytic and anti-depressant properties.

    Mechanism of Action

    Diphenhydramine predominantly works via the antagonism of H1 (Histamine 1) receptors . Such H1 receptors are located on respiratory smooth muscles, vascular endothelial cells, the gastrointestinal tract (GIT), cardiac tissue, immune cells, the uterus, and the central nervous system (CNS) neurons . When the H1 receptor is stimulated in these tissues it produces a variety of actions including increased vascular permeability, promotion of vasodilation causing flushing, decreased atrioventricular (AV) node conduction time, stimulation of sensory nerves of airways producing coughing, smooth muscle contraction of bronchi and the GIT, and eosinophilic chemotaxis that promotes the allergic immune response . Ultimately, diphenhydramine functions as an inverse agonist at H1 receptors, and subsequently reverses effects of histamine on capillaries, reducing allergic reaction symptoms . Moreover, since diphenhydramine is a first-generation antihistamine, it readily crosses the blood-brain barrier and inversely agonizes the H1 CNS receptors, resulting in drowsiness, and suppressing the medullary cough center . Furthermore, H1 receptors are similar to muscarinic receptors .

    Pharmacodynamics

    Diphenhydramine has anti-histaminic (H1-receptor), anti-emetic, anti-vertigo and sedative and hypnotic properties . The anti-histamine action occurs by blocking the spasmogenic and congestive effects of histamine by competing with histamine for H1 receptor sites on effector cells, preventing but not reversing responses mediated by histamine alone . Such receptor sites may be found in the gut, uterus, large blood vessels, bronchial muscles, and elsewhere . Anti-emetic action is by inhibition at the medullary chemoreceptor trigger zone . Anti-vertigo action is by a central antimuscarinic effect on the vestibular apparatus and the integrative vomiting center and medullary chemoreceptor trigger zone of the midbrain .

    Metabolism

    Diphenhydramine undergoes rapid and extensive first-pass metabolism . In particular, two successive N-demethylations occur wherein diphenhydramine is demethylated to N-desmethyldiphenhydramine (the N-desmethyl metabolite) and then this metabolite is itself demethylated to N,N-didesmethyldiphenhydramine (the N,N-didesmethyl metabolite) . Subsequently, acetyl metabolites like N-acetyl-N-desmethyldiphenhydramine are generated via the amine moiety of the N,N-didesmethyl metabolite . Additionally, the N,N-didesmethyl metabolite also undergoes some oxidation to generate the diphenylmethoxyacetic acid metabolite as well . The remaining percentage of a dose of administered diphenhydramine is excreted unchanged . The metabolites are further conjugated with glycine and glutamine and excreted in urine . Moreover, studies have determined that a variety of cytochrome P450 isoenzymes are involved in the N-demethylation that characterizes the primary metabolic pathway of diphenhydramine, including CYP2D6, CYP1A2, CYP2C9, and CYP2C19 . In particular, CYP2D6 demonstrates higher affinity catalysis with the diphenhydramine substrate than the other isoenzymes identified .

    Absorption

    Diphenhydramine is quickly absorbed after oral administration with maximum activity occurring in approximately one hour . The oral bioavailability of diphenhydramine has been documented in the range of 40% to 60%, and peak plasma concentration occurs about 2 to 3 hours after administration .

    Toxicity

    Overdose is expected to result in effects similar to the adverse effects that are ordinarily associated with the use of diphenhydramine, including drowsiness, hyperpyrexia, and anticholinergic effects, among others . Additional symptoms during overdose may include mydriasis, fever, flushing, agitation, tremor, dystonic reactions, hallucinations and ECG changes . Large overdose may cause rhabdomyolysis, convulsions, delirium, toxic psychosis, arrhythmias, coma and cardiovascular collapse . Moreover, with higher doses, and particularly in children, symptoms of CNS excitation including hallucinations and convulsions may appear; with massive doses, coma or cardiovascular collapse may follow . Although diphenhydramine has been in widespread use for many years without ill consequence, it is known to cross the placenta and has been detected in breast milk . This medication should therefore only be used when the potential benefit of treatment to the mother exceeds any possible hazards to the developing fetus or suckling infant . Pharmacokinetic studies indicate no major differences in the distribution or elimination of diphenhydramine compared to younger adults . Nevertheless, diphenhydramine should be used with caution in the elderly, who are more likely to experience adverse effects . Avoid use in elderly patients with confusion .

    Indication

    Diphenhydramine is a first-generation histamine H1 receptor antagonist (H1 antihistamine) that is widely available as a non-prescription, over-the-counter (OTC) medication. As an OTC medication, diphenhydramine is typically formulated as tablets and creams indicated for use in treating sneezing, runny nose, itchy/watery eyes, itching of nose or throat, insomnia, pruritis, urticaria, insect bites/stings, allergic rashes, and nausea . Additionally, when the use of oral diphenhydramine is impractical, there are also prescription-only formulations such as diphenhydramine injection products that are effective in adults and pediatric patients (other than premature infants and neonates) for: i) the amelioration of allergic reactions to blood or plasma, in anaphylaxis as an adjunct to epinephrine and other standard measures after acute allergic reaction symptoms have been controlled, and for other uncomplicated allergic conditions of the immediate type when oral therapy is impossible or contraindicated ; ii) the active treatment of motion sickness ; and iii) use in parkinsonism when oral therapy is impossible or contraindicated, as follows: parkinsonism in the elderly who are unable to tolerate more potent agents; mild cases of parkinsonism in other age groups, and in other cases of parkinsonism in combination with centrally acting anticholinergic agents .

    Half-life

    The elimination half-life ranges from 2.4-9.3 hours in healthy adults . The terminal elimination half-life is prolonged in liver cirrhosis .

    Protein Binding

    Some prescribing information records the protein binding of diphenhydramine as approximately 78% while others have suggested the medication is about 80 to 85% bound to plasma proteins.

    Elimination

    The metabolites of diphenhydramine are conjugated with glycine and glutamine and excreted in urine . Only about 1% of a single dose is excreted unchanged in urine . The medication is ultimately eliminated by the kidneys slowly, mainly as inactive metabolites .

    Volume of Distribution

    Diphenhydramine is widely distributed throughout the body, including the CNS . Following a 50 mg oral dose of diphenhydramine, the volume of distribution is in the range of 3.3 - 6.8 l/kg .

    Clearance

    Values for plasma clearance of a 50 mg oral dose of diphenhydramine has been documented as lying in the range of 600-1300 ml/min .

    Receptor Profile

    Receptor Actions

    Agonists
    H1 histamine receptor inverse agonist
    Antagonists
    Muscarinic acetylcholine receptor antagonist (M1-M5)
    Inhibitors
    weak serotonin reuptake inhibition
    Other
    sodium channel blockade
    voltage-gated potassium channel blockade

    Receptor Binding

    Muscarinic acetylcholine receptor M2 antagonist
    Histamine N-methyltransferase inhibitor

    History & Culture

    1943–1946

    Diphenhydramine was discovered in 1943 by chemist George Rieveschl and his student Fred Huber during research into muscle relaxants at the University of Cincinnati. Huber performed the initial synthesis, after which Rieveschl partnered with pharmaceutical company Parke-Davis to conduct further testing. The company subsequently licensed the patent from Rieveschl, and in 1946 diphenhydramine became the first antihistamine to receive approval from the United States Food and Drug Administration for prescription use. The compound was initially available only by prescription, remaining so for several decades. It was not until the 1980s that diphenhydramine received approval for over-the-counter sale, making it widely accessible to the general public as a sleep aid and allergy medication under various brand names, most notably Benadryl.

    1960s–present

    During the 1960s, researchers discovered that diphenhydramine inhibits the reuptake of the neurotransmitter serotonin. This finding prompted investigations into compounds with similar pharmacological mechanisms but fewer adverse effects. This line of research ultimately contributed to the development of selective serotonin reuptake inhibitors, a class of antidepressants that would come to dominate psychiatric treatment. Fluoxetine, later marketed as Prozac, emerged from this research trajectory and became one of the most widely prescribed antidepressants in the world.

    2020–present

    In 2020, a dangerous trend dubbed the "Benadryl Challenge" emerged on the social media platform TikTok. The challenge encouraged participants to deliberately consume excessive quantities of diphenhydramine in order to induce and film the resulting deliriant effects. This trend has been implicated in multiple hospitalizations and at least two deaths. A related phenomenon involves the "Hat Man," a shadowy figure commonly reported by individuals experiencing diphenhydramine-induced delirium. Typically described as a dark silhouette wearing a wide-brimmed hat, this entity has become both an internet meme and a contemporary urban legend, frequently discussed in online communities dedicated to deliriant experiences.

    Effect Profile

    Curated + 444 Reports
    Psychedelic 7.0

    Strong visuals, auditory effects, and body load with mild headspace

    Visual Intensity×3
    10103.5
    Headspace Depth×3
    53.51.2
    Auditory Effects×1
    10104.6
    Body Load / Somatic Effects×1
    95.63.1
    Catalog Erowid BlueLight
    Based on reports from:
    Effect Index Nightmares in Eiriel — Sedric Lightbury Effect Index I Slept for 27 Hours — Julie Erowid Experience Reports PsychonautWiki Diphenhydramine

    Tolerance & Pharmacokinetics

    drugs.wiki

    Tolerance Decay

    Full tolerance 1d Half tolerance 10d Baseline ~18d

    Cross-Tolerances

    Doxylamine
    30% ●○○
    Hydroxyzine
    30% ●○○

    Experience Report Analysis

    Erowid BlueLight
    363 Reports
    1996–2025 Date Range
    132 With Age Data
    31 Effects Detected

    Demographics

    Gender Distribution

    Age Distribution

    Reports Over Time

    Effect Analysis

    Erowid + Bluelight

    Effects aggregated from 413 experience reports (363 Erowid + 81 Bluelight)

    413 Reports
    142 Effects Detected
    31 Positive
    87 Adverse
    24 Neutral

    Effect Sentiment Distribution

    Confidence Distribution

    Positive Effects 31

    Sedation 46.5% 90%
    Anxiety Suppression 46.2% 84%
    Stimulation 33.9% 70%
    Vivid Dreams 30.0% 86%
    Music Enhancement 23.2% 88%
    Euphoria 20.6% 83%
    Color Enhancement 19.9% 79%
    Focus Enhancement 19.6% 70%
    Tactile Enhancement 19.4% 80%
    Empathy 15.7% 70%
    Contentment 12.0% 76%
    Body High 10.4% 85%
    Awe 10.0% 70%
    Drowsiness 6.0% 85%
    Introspection 5.5% 74%
    Geometric Imagery 4.0% 85%
    Warping 4.0% 80%
    Lightness 4.0% 80%
    Emotional Openness 4.0% 68%
    Creativity Enhancement 2.2% 70%

    Adverse Effects 87

    Entity Imagery 62.0% 87%
    Heaviness 46.0% 87%
    Body Load 42.0% 82%
    Fear 40.0% 89%
    Confusion 32.5% 89%
    Thought Disorganization 30.0% 84%
    Ataxia 30.0% 86%
    Dry Mouth 30.0% 83%
    Memory Suppression 24.5% 89%
    Depersonalization 24.0% 78%
    Amnesia 24.0% 88%
    Nausea 21.8% 84%
    Panic 20.0% 88%
    Motor Impairment 18.4% 82%
    Tremor 18.0% 83%
    Frequent Urination 16.0% 81%
    Paranoia 14.0% 86%
    Temporal Disorientation 14.0% 85%
    Delusion 14.0% 85%
    Increased Heart Rate 12.7% 70%

    Dose-Response Correlation

    How effect frequency changes across dose levels

    View data table
    Effect Threshold (n=24) Light (n=31) Common (n=73) Strong (n=56) Heavy (n=49)
    Anxiety Suppression 45.8% 29.0% 45.2% 55.4% 59.2%
    Sedation 54.2% 48.4% 53.4% 57.1% 36.7%
    Confusion 16.7% 19.4% 28.8% 41.1% 51.0%
    Visual Distortions 29.2% 41.9% 50.7% 33.9% 40.8%
    Auditory Effects 0% 32.3% 32.9% 44.6% 38.8%
    Open-Eye Visuals 20.8% 29.0% 39.7% 42.9% 42.9%
    Hospital 20.8% 19.4% 6.8% 21.4% 38.8%
    Stimulation 37.5% 38.7% 35.6% 35.7% 26.5%
    Tactile Enhancement 16.7% 35.5% 15.1% 26.8% 28.6%
    Memory Suppression 8.3% 19.4% 28.8% 33.9% 26.5%
    Music Enhancement 33.3% 32.3% 27.4% 30.4% 14.3%
    Euphoria 33.3% 29.0% 21.9% 26.8% 10.2%
    Color Enhancement 16.7% 32.3% 20.5% 26.8% 28.6%
    Nausea 16.7% 9.7% 24.7% 30.4% 20.4%
    Focus Enhancement 25.0% 25.8% 23.3% 23.2% 14.3%

    Subjective Effect Ontology

    Experience Reports

    Structured effect tags extracted from 444 Erowid & Bluelight experience reports using a controlled vocabulary of 220+ canonical effects across 15 domains.

    Auditory

    auditory distortions 122 33.6%

    Cognitive

    confusion 134 32.8% memory suppression 101 24.7%

    Emotional

    anxiety suppression 191 44.8%

    Motor

    sedation 192 46.6% stimulation 123 33.9%

    Visual

    visual distortions 140 32.2% open eye visuals 130 31.1%

    8 unique effects extracted · Derived from Erowid & Bluelight reports

    Dose–Effect Mapping

    Experience Reports

    How reported effects shift across dose tiers, based on 363 experience reports.

    Oral dose range: 200.0–600.0 mg (median 325.0 mg)
    Effect Threshold (n=24) Light (n=31) Common (n=73) Strong (n=56) Heavy (n=49)
    anxiety suppression
    46%
    29%
    45%
    55%
    59%
    		↑
    sedation
    54%
    48%
    53%
    57%
    37%
    		↓
    confusion
    17%
    19%
    29%
    41%
    51%
    		↑
    visual distortions
    29%
    42%
    51%
    34%
    41%
    		↑
    auditory effects —
    32%
    33%
    45%
    39%
    		↑
    open-eye visuals
    21%
    29%
    40%
    43%
    43%
    		↑
    hospital
    21%
    19%
    7%
    21%
    39%
    		↑
    stimulation
    38%
    39%
    36%
    36%
    26%
    		↓
    tactile enhancement
    17%
    36%
    15%
    27%
    29%
    		↑
    memory suppression
    8%
    19%
    29%
    34%
    26%
    		↑
    music enhancement
    33%
    32%
    27%
    30%
    14%
    		↓
    euphoria
    33%
    29%
    22%
    27%
    10%
    		↓
    color enhancement
    17%
    32%
    20%
    27%
    29%
    		↑
    nausea
    17%
    10%
    25%
    30%
    20%
    		↑
    focus enhancement
    25%
    26%
    23%
    23%
    14%
    		↓
    dissociation
    12%
    26%
    15%
    20%
    18%
    		↑
    closed-eye visuals
    12%
    23%
    12%
    11%
    8%
    		↓
    increased heart rate
    17%
    		—
    10%
    12%
    22%
    		↑
    empathy
    17%
    19%
    22%
    11%
    4%
    		↓
    motor impairment
    12%
    16%
    22%
    20%
    6%
    		↓

    Showing top 20 of 33 effects

    Risk Escalation

    Sentiment Analysis

    Average frequency of positive vs adverse effects across dose tiers

    Threshold n=24
    9 positive 22.2% 7 adverse 18.5%
    Light n=31
    9 positive 25.8% 8 adverse 14.9%
    Common n=73
    9 positive 20.5% 11 adverse 17.1%
    Strong n=56
    10 positive 20.9% 12 adverse 19.1%
    Heavy n=49
    11 positive 13.9% 12 adverse 19.9%
    View effect breakdown

    Adverse Effects

    Effect Threshold (n=24) Light (n=31) Common (n=73) Strong (n=56) Heavy (n=49) Change
    Anxiety Suppression
    46%
    29%
    45%
    55%
    59%
    		+29%
    Confusion
    17%
    19%
    29%
    41%
    51%
    		+205%
    Memory Suppression
    8%
    19%
    29%
    34%
    26%
    		+219%
    Nausea
    17%
    10%
    25%
    30%
    20%
    		+22%
    Increased Heart Rate
    17%
    		—
    10%
    12%
    22%
    		+34%
    Motor Impairment
    12%
    16%
    22%
    20%
    6%
    		-51%
    Seizure —
    10%
    4%
    4%
    16%
    		+68%
    Pupil Dilation
    12%
    6%
    6%
    11%
    14%
    		+14%
    Psychosis — —
    4%
    5%
    10%
    		+148%
    Headache —
    10%
    7%
    7%
    4%
    		-57%
    Muscle Tension — —
    8%
    5%
    		— -34%
    Sweating — — —
    4%
    4%
    		+13%
    Jaw Clenching — — — —
    4%
    		0%

    Positive Effects

    Effect Threshold (n=24) Light (n=31) Common (n=73) Strong (n=56) Heavy (n=49) Change
    Stimulation
    38%
    39%
    36%
    36%
    26%
    		-29%
    Tactile Enhancement
    17%
    36%
    15%
    27%
    29%
    		+71%
    Music Enhancement
    33%
    32%
    27%
    30%
    14%
    		-57%
    Euphoria
    33%
    29%
    22%
    27%
    10%
    		-69%
    Color Enhancement
    17%
    32%
    20%
    27%
    29%
    		+71%
    Focus Enhancement
    25%
    26%
    23%
    23%
    14%
    		-42%
    Empathy
    17%
    19%
    22%
    11%
    4%
    		-75%
    Body High
    12%
    13%
    15%
    16%
    8%
    		-34%
    Introspection —
    6%
    4%
    9%
    6%
    		-6%
    Pain Relief
    8%
    		— — —
    6%
    		-26%
    Creativity Enhancement — — —
    4%
    6%
    		+69%

    Dosage Distribution

    Dose distribution from experience reports

    Median: 325.0 mg IQR: 200.0–600.0 mg n=231

    Real-World Dose Distribution

    62K Doses

    From 575 individual dose entries

    Oral (n=486)

    Median: 250.0mg 25th: 75.0mg 75th: 500.0mg 90th: 750.0mg
    mg/kg median: 3.107 mg/kg 75th: 7.052

    Common Combinations

    Most co-occurring substances in experience reports

    Form / Preparation

    Most common forms and preparations reported

    Body-Weight Dosing

    Dose relative to body weight from reports with weight data

    Median: 4.902 mg/kg IQR: 2.755–8.824 mg/kg n=230

    Redose Patterns

    Redosing behavior across 298 reports

    20.5% Redosed
    1.3 Avg Doses
    50m Median Interval
    Read full reports on Erowid →

    Legal Status

    Country Status Notes
    Australia Over-the-counter Widely available without prescription at pharmacies.
    Austria Over-the-counter Available without prescription. Sold under brand names such as Calmaben in 50mg tablet formulations.
    Belgium Over-the-counter Available without prescription. Marketed under brand names including Nustasium in 50mg tablet form.
    Brazil Pharmacy-restricted Not scheduled as a controlled substance but restricted to pharmacy sales. Availability as over-the-counter or prescription-only depends on formulation and concentration. Pure injectable forms are restricted to hospital environments for intravenous or intramuscular administration.
    Bulgaria Over-the-counter Available without prescription. Sold under brand names including Calmaben and combination products such as APAP Night.
    Canada Over-the-counter Freely available without prescription throughout the country.
    Germany Over-the-counter Freely available without prescription at pharmacies throughout the country.
    New Zealand Pharmacy-only/Prescription Controlled under the 'Sedating Antihistamines' category. Restricted to pharmacy sales or available by prescription, with more stringent controls than most other countries.
    Poland Uncontrolled (age-restricted) Not classified as a controlled substance under Polish law. Sales restricted to individuals 18 years or older. Primarily found in sleep medication formulations rather than as a standalone antihistamine product.
    South Africa Schedule 2 (OTC) Available over-the-counter under Schedule 2 of the South African classification system. Sold under brand names such as Betasleep in 50mg capsule formulations.
    United Kingdom Over-the-counter (age-restricted) Available without prescription at pharmacies and retail outlets. Typically not sold to individuals under 16 years of age, though enforcement varies by retailer.
    United States Unscheduled (OTC) Widely available over-the-counter as an FDA-approved medication. Legal to purchase, possess, and consume without a prescription or license. Sales for human consumption are regulated by the FDA, with each formulation requiring specific approval.
    Zambia Controlled substance Illegal to possess and sell. The Zambian Drug Enforcement Commission actively enforces restrictions, and foreign nationals have been detained and charged with drug trafficking offenses for possessing over-the-counter medications containing diphenhydramine, even in small quantities. Passports may be confiscated and imprisonment is possible.

    References

    Data Sources

    Cited References

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