Diphenhydramine (DPH, also sold as Benadryl) is an antihistamine with anticholinergic and sedative effects. It is found in both OTC and prescription drugs used in the treatment of allergy-related symptoms, some cold and flu symptoms, insomnia, motion sickness and Parkinson's disease. It has also been researched for many other medicinal uses, including the treatment of OCD.

It has also seen recreational use as a depressant or deliriant drug, with high doses inducing vivid hallucinations which users are often unable to distinguish from reality. Most recreational users find the drug causes a negative experience, due to the profusely negative experience it tends to provide - along with the large amount of negative physical side-effects.


DPH was first discovered in 1943 in the USA during a research project fielded around relieving muscle spasms - and in 1946 it became the first prescription antihistamine approved in the USA - therefore, it is known as a 'first generation antihistamine.'

In the 1960s, it was discovered that DPH inhibited the reuptake of serotoninA monoamine neurotransmitter, biochemically derived from tryptophan, that is primarily found in the gastrointestinal (GI) tract, platelets, and central nervous system (CNS) of humans and animals. It is a well-known contributor to feelings of well-being., which, through research of chemicals with similar actions causing less side-effects, led to the development of the antidepressant class SSRIs which are still in heavy use today.

In the 1980s, the drug became approved for over-the-counter use as a sleep-aid (due to its sedative effect) and antihistamine.


Overdoses of DPH may cause seizures, coma and ultimately an acute overdose of DPH can be fatal through cardiovascular collapse - though the threshold for this type of reaction is relatively unknown.

DPH is primarily taken orallyRoute of administration in which the subject swallows a substance..

Therapeutic (insomnia) 50mg
Light 150-300mg
Common 250-500mg
Strong 450-700mg
Heavy 650-800mg


Onset 60-120 minutes
Total 6-8 hours
After-effects 24 hours


Though DPH causes hallucinations and distortions in the perception, which are often effects sought by the recreational users of psychedelics, these effects are often negative in nature due to the deliriant nature of the chemical and experience.


  • Increased awareness and appreciation of music
  • Dream-like experiences
  • Vivid hallucinations
  • Muscle relaxation


  • Visual distortions
  • Sound distortions


  • Dry mouth
  • Delirium
  • Confusion
  • Muscle cramps
  • Nausea
  • Loss of coordination (ataxiaLoss of motor coordination)
  • Increased heart rate
  • Photophobia
  • Flushed skin
  • Itching
  • Heart palpitations
  • Tremors
  • Vomiting
  • Stomach pain
  • Dysphoria

Chemistry and Pharmacology

DPH molecule.png

Chemical name: 2-(diphenylmethoxy)-N,N-dimethylethanamine

Diphenhydramine is an inverse agonistA substance that initiates a physiological response when combined with a receptor. of the histamine H1 receptor. It is a member of the ethanolamine class of antihistaminergic agents. By reversing the effects of histamine on the capillaries, it can reduce the intensity of allergic symptoms. Diphenhydramine also crosses the blood–brain barrier and antagonizes the H1 receptors centrally. Its effects on central H1receptors cause drowsiness.

Harm Reduction

  • Many oral diphenhydramine preparations contain analgesics such as acetaminophen/paracetamol or aspirin, which can be toxic to the liver in high doses. Some adulterants, such as paracetamol, can be removed by Cold Water Extraction.
  • Do not combine DPH with any other depressant drugs such as benzos, DXM, alcohol, or opiates
  • DPH in high doses may induce delirium, making him or her unpredictable and possible dangerous, especially in public
  • Never drive or operate heavy machinery under the influence
  • If taking high doses, it is a very good idea to have a sober tripsitter or a friend with you


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