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Diphenhydramine exhibits a nonlinear dose-response curve. Doses below 300 mg typically produce restlessness, muscle relaxation, and bodily sensations. Doses exceeding 500 mg may induce a deliriant state with fully-formed visual and auditory hallucinations. Intermediate dosing ranges frequently result in dysphoric and uncomfortable experiences. Doses approaching or exceeding 2 grams have potential for fatal outcomes, particularly when combined with stimulants or MAOI inhibitors.
Diphenhydramine acts primarily as an inverse agonist of the histamine H1 receptor, both peripherally and centrally. Its ability to cross the blood-brain barrier and inversely agonize central H1 receptors accounts for its sedative properties. It is also a potent antimuscarinic, functioning as a competitive antagonist at muscarinic acetylcholine receptors. This anticholinergic activity is thought to be primarily responsible for the deliriant effects, including delirium, sedation, and intensely realistic hallucinations, though the precise mechanism is not fully understood. Beyond these primary actions, diphenhydramine acts as an intracellular sodium channel blocker, inhibits the reuptake of serotonin, blocks voltage-gated potassium channels, acts as an inhibitor of histamine N-methyltransferase, and is a weak antagonist of the 5-HT2C receptor.
Diphenhydramine is quickly absorbed after oral administration, with peak plasma concentration occurring approximately 2 to 3 hours after dosing. Oral bioavailability is in the range of 40% to 60%, reflecting rapid and extensive first-pass metabolism. The primary metabolic pathway involves two successive N-demethylations, first to N-desmethyldiphenhydramine and then to N,N-didesmethyldiphenhydramine. The latter undergoes acetylation and oxidation to generate additional metabolites, including diphenylmethoxyacetic acid. These metabolites are further conjugated with glycine and glutamine and excreted in urine, with only about 1% of a dose excreted unchanged. The cytochrome P450 enzymes CYP2D6, CYP1A2, CYP2C9, and CYP2C19 are involved in the N-demethylation pathway, with CYP2D6 demonstrating the highest affinity for the diphenhydramine substrate. The elimination half-life ranges from 2.4 to 9.3 hours in healthy adults, though it varies considerably with age.
These combinations are considered extremely harmful and should always be avoided. Reactions to these drugs taken in combination are highly unpredictable and have a potential to cause death.
There is considerable risk of physical harm when taking these combinations, they should be avoided where possible.
These combinations are not usually physically harmful, but may produce undesirable effects, such as physical discomfort or overstimulation. Extreme use may cause physical health issues. Synergistic effects may be unpredictable. Care should be taken when choosing to use this combination.
Generally considered to have low abuse potential due to its dysphoric effects; most individuals who try recreational doses do not wish to repeat the experience. However, cases of abuse and addiction have been documented, particularly among adolescents without access to other drugs and individuals with mental health conditions such as schizophrenia who may self-administer large doses to treat extrapyramidal symptoms.
Produces dependence with chronic use, though the severity and specific withdrawal symptoms are not well-documented in the literature.
The overdose threshold is commonly held to be around 1000 milligrams, though sensitive individuals or those taking other substances may overdose at lower amounts. Doses approaching or exceeding 2 grams can be fatal. Acute poisoning can lead to cardiovascular collapse and death within 2-18 hours. No specific antidote exists; treatment is symptomatic and supportive.
| Species | Route | Value |
|---|---|---|
| rat | oral | 500 mg/kg |
High doses and overdoses can cause cardiotoxicity including arrhythmias, severe tachycardia, QT prolongation, and cardiovascular collapse; myocardial infarction and serious ventricular dysrhythmias have been reported with considerable overdosage.
Regular recreational use has been anecdotally linked to kidney and bladder damage with symptoms similar to ketamine cystitis; these reports are associated with chronic high-dose use patterns and remain unstudied.
Chronic high-dose use may cause persisting hallucinations and impairments in cognition and memory; cumulative anticholinergic use later in life has been tentatively linked to increased risk of cognitive decline and dementia.
The drug is extensively metabolized by the liver; caution is advised in individuals with hepatic impairment as half-life is prolonged in patients with chronic liver disease.
Large overdoses may cause rhabdomyolysis, particularly when excessive physical activity occurs during intoxicated states where the user cannot distinguish reality from hallucinations.
Causes psychosis and delirium at significantly higher rates than other hallucinogens such as psychedelics and dissociatives. Toxic psychosis, delirium, and complete inability to distinguish hallucinations from reality are characteristic features at recreational doses. Users may lose control of their actions and respond to delusional environments, risking injury to themselves or others. Numerous experience reports describe psychotic delirium, amnesia, and serious consequences including hospitalization and death.
High doses and overdoses have been linked to seizures and convulsions, with risk increasing particularly at overdose-level doses. Children are more susceptible to CNS excitation including convulsions at elevated doses. Benzodiazepines are recommended as first-line treatment to decrease seizure likelihood in overdose cases.
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…
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