spec sheet11 rows
Diphenhydramine is a first-generation H1 antihistamine sold over the counter for allergies and as a sleep aid, sedating at ordinary doses through histamine H1 blockade. It is also a potent muscarinic acetylcholine receptor antagonist, and at the high doses taken recreationally this antimuscarinic action produces a deliriant state of confusion, agitation, and frightening hallucinations characteristic of the anticholinergic toxidrome rather than euphoria. Toxicity is dose-dependent, with the risk of delirium, seizures, and coma rising sharply above roughly one gram, and diphenhydramine additionally blocks cardiac sodium channels, causing QRS prolongation and conduction abnormalities that may respond to sodium bicarbonate. Severe anticholinergic delirium can be reversed with centrally acting cholinesterase inhibitors such as physostigmine or, where unavailable, rivastigmine.
- Relieves allergy symptoms
- Sedation and sleep aid at normal doses
- Reduces motion sickness and itching
- Anticholinergic deliriant with sodium-channel cardiotoxicity
- Frightening, dysphoric delirium at high doses
- Racing heart and overheating
- Urinary retention and dry mouth
- Seizures and heart-rhythm disturbances in overdose
Mechanism
Diphenhydramine is a competitive at H1 receptors, which is what makes it useful for allergies and drowsy at normal doses. Its deliriant character comes from antagonism of receptors; blocking central muscarinic signaling disrupts memory, attention and the boundary between waking and dreaming, producing confusion, hallucinations and delirium at high doses. It also has some sodium-channel-blocking activity, which contributes to its cardiac toxicity in overdose.
receptor fingerprint
H1 receptorAntagonist
receptorsAntagonist
Cardiac sodium channelsBlocker
Safetyrisks and cautions, not medical advice
The recreational deliriant dose sits close to genuinely toxic territory; anticholinergic overdose brings racing heart, dangerously high body temperature, urinary retention, seizures and, via sodium-channel blockade, life-threatening heart-rhythm disturbances. The experience itself is typically frightening and dysphoric rather than pleasant, with realistic hallucinations people cannot tell from reality. It stacks dangerously with other anticholinergics (the total 'anticholinergic burden') and with other sedatives. Not medical advice.
Interactionsdocumented pairs only, not exhaustive
Diphenhydramine carries two separate interaction problems. The obvious one is additive: alcohol, benzodiazepines, opioids, z-drugs and sedating antidepressants all deepen its sedation and psychomotor impairment, and its own antimuscarinic activity stacks with tricyclic antidepressants, low-potency antipsychotics, oxybutynin and scopolamine. In older adults that combined anticholinergic burden produces confusion, delirium, urinary retention and constipation, which is much of the reason diphenhydramine sits on deprescribing lists.
The less obvious problem is metabolic. Diphenhydramine is a potent competitive inhibitor of CYP2D6. In people who are extensive metabolisers it roughly halves metoprolol clearance and prolongs metoprolol's effects on heart rate and blood pressure, an effect measured as larger in women than in men. The same inhibition applies to other CYP2D6 substrates with narrow margins, including tricyclics and some antiarrhythmics, and to codeine, whose conversion to morphine it can block.
MAO inhibitors prolong and intensify the anticholinergic effects, a combination that is generally avoided.
Checking a whole stack? Run it through interactions + stacks.
Subjective profileweighing the evidence above
Fine for allergies and the occasional bad night, though better sleep aids exist. Taken for a high it is one of the worst deals in recreational pharmacology: the delirium is frightening rather than pleasant, and the same doses bring seizures and sodium-channel cardiotoxicity.
Resources
This entry is here for reference.
Research
- 2000first citedDose-dependent toxicity of diphenhydramine overdose.
- 2025most recentTransdermal rivastigmine as a therapeutic option in severe diphenhydramine-induced anticholiner…
- 1.Status epilepticus and wide-complex tachycardia secondary to diphenhydramine overdose
- 2.Quantitative prediction of histamine H1 receptor occupancy by the sedative and non-sedative antagonists in the human central nervous system based on systemic exposure and preclinical data
- 3.Dose-dependent toxicity of diphenhydramine overdose.
- 4.Transdermal rivastigmine as a therapeutic option in severe diphenhydramine-induced anticholinergic toxicity: A case report and literature review.
- 5.Left bundle branch block morphology on electrocardiogram after massive diphenhydramine overdose.
- 6.Over-the-counter Psychosis: A Systematic Review of the Misuse of Antihistamines, Cough Medicines, and Decongestants and the Risk of Developing Psychosis.
6 listed here; entry last updated August 2026
Reviews
My notesprivate to this device
FAQ
Is a diphenhydramine trip fun?
Almost universally reported as unpleasant. It is a deliriant, so you get confusion, frightening realistic hallucinations and a rough physical hangover rather than euphoria.
Why is it considered dangerous?
The recreational dose is close to a toxic one, and overdose can cause overheating, seizures and dangerous heart rhythms from its sodium-channel effects.
Is it addictive?
It is not classically addictive the way stimulants are, but some people misuse it habitually for sleep, and the anticholinergic effects can accumulate harmfully.
Adverse effects
- Frightening, dysphoric delirium at high doses
- Racing heart and overheating
- Urinary retention and dry mouth
- Seizures and heart-rhythm disturbances in overdose