Pharmacology - Diphenhydramine

Discussion in 'Deliriant antihistamines' started by darkglobe, Mar 8, 2007.

  1. darkglobe

    darkglobe Silver Member

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    Dec 12, 2006
    from U.K.
    The majority of this information is used without permission from external sources. I have merely compiled it into one in-depth overview of how Diphen achieves the effects that it does, and also how it works medicinally. I have also carried out a quick comparison to Dimenhydrinate (a remotely similar drug, but with almost identical effects).

    This first post is the overview of the chemical, it's components and some bits of background info on the drug. I will follow this post with more detailed information as I learn it. I don't post stuff I do not yet understand, and as a result spend days on end learning more and more about neuroscience and brain chemistry.

    I hope this is an informative yet still mildly interesting idea.


    Chemical IUPAC Name: 2-benzhydryloxy-N,N-dimethyl-ethanamine
    Chemical Formula: C17H21NO
    Chemical Structure:

    ndication For the treatment of symptoms associated with Vertigo/Meniere's disease, nausea and vomiting, motion sickness and insect bite. [Also used as a sedative.

    Basic Pharmacology:
    Diphenhydramine is an antihistamine of the ethanolamine class. Ethanolamine antihistamines have significant antimuscarinic activity and produce marked sedation in most patients. In addition to the usual allergic symptoms, the drug also treats irritant cough and nausea, vomiting, and vertigo associated with motion sickness. It also is used commonly to treat drug-induced extrapyramidal symptoms as well as to treat mild cases of Parkinson's disease. Rather than preventing the release of histamine, as do cromolyn and nedocromil, diphenhydramine competes with free histamine for binding at HA-receptor sites. Diphenhydramine competitively antagonizes the effects of histamine on HA-receptors in the GI tract, uterus, large blood vessels, and bronchial muscle. Ethanolamine derivatives have greater anticholinergic activity than do other antihistamines, which probably accounts for the antidyskinetic action of diphenhydramine. This anticholinergic action appears to be due to a central antimuscarinic effect, which also may be responsible for its antiemetic effects, although the exact mechanism is unknown.
    Mechanism Of Action: Diphenhydramine competes with free histamine for binding at HA-receptor sites. This antagonizes the effects of histamine on HA-receptors, leading to a reduction of the negative symptoms brought on by histamine HA-receptor binding.
    The drug targets histamine receptor H1. The drugs main process works because in peripheral tissues, the H1 subclass of histamine receptors mediates the contraction of smooth muscles, increase in capillary permeability due to contraction of terminal venules, and catecholamine release from adrenal medulla, as well as mediating neurotransmission in the central nervous system. Okay, I admit it, only about 4 words in that part I actually understand, but it's good for those who understand neuroscience to learn how they get their drug effects. And if anybody says scientists don't do drugs I can quite easily and happily argue :D
    Toxicity: LD50=500 mg/kg (Orally in rats, so no guarantees!)
    Biotransformation: Hepatic and renal. Basically, the drug is metabolised in the kidney and liver (hepatic is liver, right?).

    Info still to come: Differences to dimenhydrinate, why uncoated Diphen tablets hurt like fuck if you leave them in your mouth too long, more on how the deleriant effects are created and finally how different routes of administration affect the experience.

    PS: I've just unearthed some info linking Diphen with cognitive decline. Will post in the Dangers thread if you're interested.